two parts, one endoskeletal and cartilaginous, corresponding with the
pectoral girdle of Elasmobranchs, and one exoskeletal and formed of
membrane bones, corresponding with the clavicular bones of
Teleosteans. The pelvic fins are always abdominal. The fins often, as
in _Polypterus_, have spines (fulcra) attached to their anterior
borders.

The order Ganoidei may be divided into three suborders:

     (1) CHONDROSTEI. Living genera _Acipenser_, _Scaphirhynchus_,
     _Polyodon_ and _Psephurus_.

     (2) CROSSOPTERYGII. Living genera _Polypterus_ and
     _Calamoichthys_.

     (3) HOLOSTEI. Living genera _Lepidosteus_ and _Amia_.

_Suborder_ (1). CHONDROSTEI.

The skull is immovably fixed to the vertebral column. By far the
greater part of the skeleton is cartilaginous. The notochord is
persistent and unconstricted, its sheath is membranous, but
cartilaginous neural and haemal arches are developed. Intercalary
pieces (interdorsalia) occur between the neural arches, and similar
pieces (interventralia) between the haemal arches. The cranium is
covered with membrane bone, and teeth are but slightly developed. The
tail is heterocercal. Gill rays occur on the hyoid arch, and the gills
are protected by a bony operculum attached to the hyomandibular. The
skin (1) may be almost or quite naked, (2) may carry bony plates
arranged in rows, or may be covered (3) with rhomboidal scales, or (4)
partly with rhomboidal, partly with cycloidal scales.

_Suborder_ (2). CROSSOPTERYGII.

The exoskeleton has the form of cycloidal or rhomboidal scales. The
condition of the vertebral column differs in the different genera.
Sometimes, as in _Polypterus_, there are well-developed ossified
vertebrae; sometimes, as in many extinct forms, the notochord persists
and is unconstricted. The tail may be diphycercal or heterocercal. The
pectoral and sometimes the pelvic fins consist of an endoskeletal axis
bearing a fringe of dermal rays.

_Suborder_ (3). HOLOSTEI.

The exoskeleton has the form of cycloidal or rhomboidal scales. The
notochord is constricted and its sheath is segmented and ossified,
forming distinct vertebrae, which are generally biconcave, sometimes
opisthocoelous (_Lepidosteus_). The cartilaginous cranium is largely
replaced by bone, and in connection with it we find not only membrane
bone, but cartilage bone, as the basi-occipital, exoccipitals, and
pro-otic are ossified. The tail is heterocercal. The suspensorium
resembles that of Teleosteans, consisting of a proximal ossification,
the hyomandibular, which is movably articulated to the skull and a
distal ossification, the symplectic. The two are separated by some
unossified cartilage. The cartilaginous upper and lower jaws are to a
great extent surrounded and replaced by a series of membrane bones.

_Order_ IV. TELEOSTEI.

The exoskeleton is sometimes absent but generally consists of
overlapping cycloid or ctenoid scales. Bony plates are sometimes
present, as in the Siluridae, or the body may be encased in a complete
armour of calcified plates, as in _Ostracion_. Enamel is however never
present, and the plates are entirely mesodermal. The skeleton is bony,
but in the skull much cartilage generally remains. The vertebral
centra are usually deeply biconcave, and the tail is of the masked
heterocercal type distinguished as _homocercal_. In the skull the
occipital region is always completely ossified, while the sphenoidal
region is generally less ossified. The skull has usually a very large
number of membrane bones developed in connection with it. The teeth
vary much in character in the different members of the order, but are
as a rule numerous and pointed, and are ankylosed to the bone. The
suspensorium is hyostylic and the jaws have much the same arrangement
as in the Holostei. There are five pairs of branchial arches, of which
all except the last bear gill rays. A series of dermal opercular bones
is developed in connection with these arches. The pectoral girdle
consists almost entirely of dermal clavicular bones. The pelvic girdle
has disappeared, its place being taken by the enlarged and ossified
dermal fin-rays of the pelvic fins.

The group includes the vast majority of living fish (see p. 33).

_Order_ V. DIPNOI.

The exoskeleton is of two types; dermal bones are largely developed in
the head region, while the tail and posterior part of the body may be
naked or may be covered with overlapping scales. The cranium remains
chiefly cartilaginous, the palato-pterygo-quadrate bar is fused with
the cranium, and the suspensorium is autostylic. The gill clefts are
feebly developed and open into a cavity covered by an operculum. The
notochord is persistent and unconstricted, and the limbs are
archipterygia. The pelvic fins are without claspers.

_Suborder_ (1). SIRENOIDEI[33].

The head has well developed membrane bones. The trunk is covered with
overlapping scales and bears no bony plates. Three pairs of teeth are
present, two in the upper and one in the lower jaw, the two principal
pairs of teeth are borne on the palato-pterygoids and splenials, while
the third pair are found in the vomerine region. The tail is
diphycercal in living forms. In the extinct Dipteridae it is
heterocercal. The pectoral girdle includes both membrane and cartilage
bones. The pelvic girdle consists of a single bilaterally symmetrical
piece of cartilage.

This suborder is represented by the living genera _Ceratodus_,
_Protopterus_ and _Lepidosiren_, and among extinct forms by the
Dipteridae and others.

_Suborder_ (2). ARTHRODIRA.

Bony plates are developed not only on the head but also on the
anterior part of the trunk, where they consist of a dorsal, a ventral,
and a pair of lateral plates which articulate with the cranial shield.
The posterior part of the trunk is naked. The tail is diphycercal. The
jaws are shear-like, and their margins are usually provided with
pointed teeth whose bases fuse with the tissue of the jaw and
constitute dental plates. There seem to have been three pairs of these
plates, arranged as in the Sirenoidei, the principal ones in the upper
jaw being borne on the palato-pterygoids. Small pelvic fins are
present, but pectoral fins are unknown.

The Arthrodira occur chiefly in beds of Devonian and Carboniferous
age. Two of the best known genera are _Coccosteus_ from the European
Devonian and _Dinichthys_, a large predatory form from the lower
Carboniferous of Ohio.

FOOTNOTES:

[29] See p. 79.

[30] For this and other groups of extinct fish see A. Smith Woodward,
_Catalogue of Fossil Fish in the British Museum_, Parts I.-III.
London, 1889-95.

[31] See p. 127.

[32] See B. Dean, _J. Morphol._ vol. IX. pp. 87-114, 1894, and _Nat.
Sci._ vol. VIII. p. 245, 1896.

[33] A. Günther, _Phil. Trans._ vol. 161, Part II. 1871, p. 511. T.H.
Huxley, "On Ceratodus and the classification of fishes," _P.Z.S._
1876, p. 24.

CHAPTER VI.

THE SKELETON OF THE DOGFISH[34].

_Scyllium canicula._

I. EXOSKELETON.

The exoskeleton of the dogfish is mainly composed of placoid scales,
each of which consists of a little bony base imbedded in the skin,
bearing a small backwardly-directed spine formed of dentine capped
with enamel. The scales are larger on the dorsal than on the ventral
surface, and on the jaws they are specially large and regularly
arranged in rows, there forming the teeth. The margins of the jaws or
lips are without scales.

A second exoskeletal structure is found in the fins, all of which,
both paired and unpaired, have, in addition to their cartilaginous
endoskeleton, large numbers of long slender horny fibres, the
fin-rays, which are of exoskeletal origin.

II. ENDOSKELETON.

The endoskeleton of the dogfish consists almost entirely of cartilage,
which however may become calcified in places, e.g. the centrum of each
vertebra is lined by a layer of calcified tissue.

The endoskeleton is divisible into an =axial= portion consisting of
the vertebral column, skull, and skeleton of the median fins, and an
=appendicular= portion consisting of the skeleton of the paired fins
and their girdles.

1. THE AXIAL SKELETON.

A. THE VERTEBRAL COLUMN AND RIBS.

The vertebral column consists of a series of some hundred and thirty
vertebrae, each of which is united with its predecessor and successor
in such a way as to allow a large amount of flexibility.

These vertebrae are developed round an unsegmented rod, the
=notochord=, which forms the axial support of the embryo. The
notochord remains continuous throughout the whole vertebral column,
but is greatly constricted opposite the middle of each vertebra, and
thus rendered moniliform. The vertebrae are divided into two groups,
an anterior group of trunk vertebrae, and a posterior group of caudal
or tail vertebrae.

A typical vertebra consists of a middle portion, the =centrum=, a
dorsal portion, the =dorsal= or =neural arch=, which surrounds the
spinal cord, and a ventral portion, the =ventral= or =haemal arch=,
which similarly encloses a space.

The tail vertebrae of the dogfish have this typical arrangement, the
trunk vertebrae have the haemal arches modified.

Each =centrum= is a short cylinder of cartilage surrounding an
hourglass-shaped cavity occupied by the notochord. The =neural arches=
are composed of three separate elements, the =vertebral neural plates=
(basidorsalia), =intervertebral neural plates= (interdorsalia), and
=neural spines= (supradorsalia).

The =vertebral neural plates= are in the adult fused with their
respective centra, and are notched behind for the exit of the ventral
(motor) roots of the spinal nerves. The =intervertebral neural plates=
are polygonal pieces alternating with the vertebral neural plates;
they are notched behind, but at a more dorsal level than are the
vertebral neural plates, for the exit of the dorsal or sensory roots
of the spinal nerves.

The =neural spines= are small patches of cartilage filling up the gaps
between the dorsal ends of the neural plates.

The =haemal arches= (basiventralia) differ much in the trunk and tail
portions of the vertebral column. In the trunk portion the centra are
flattened below, and the two halves of the haemal arch diverge from
one another as blunt =ventri-lateral processes= to
which short cartilaginous rods, the =ribs=, are attached. Further back
at about vertebra 37, the two halves of the haemal arch project
downwards and meet forming a complete arch. Further back still,
towards the hind end of the tail, the haemal arches bear median
=haemal spines= (ventrispinalia).

B. THE SKULL.

The skull of the dogfish remains cartilaginous throughout the life of
the animal, and has consequently a far more simple structure than have
the skulls of higher animals, in which complication has been produced
by the development of bone.

The skull consists of the following parts:--

(1) a dorsal portion, the =cranium=, which lodges the brain, and to
the sides of which the capsules of the auditory and olfactory sense
organs are united. The cranium may be compared to an unsegmented
continuation of the vertebral column;

(2) a number of ventral structures, disconnected or only loosely
connected with the cranium. These together constitute the =visceral
skeleton= forming the jaws and supporting the gills.

(1) THE CRANIUM.

The =Cranium= is an oblong box, with a flattened floor and a more
irregular roof. Its sides are expanded in front owing to the
olfactory capsules, and behind owing to the auditory capsules, while
in the middle they are deeply hollowed to form the orbits.

(_a_) On the dorsal surface of the cranium the following points should
be noticed. First at the anterior end, the large thin-walled =nasal=
or =olfactory capsules= (fig. 6, 1), each of which is drawn out into a
narrow cartilaginous process.

The olfactory capsules have no ventral walls, and are separated from
one another by the =internasal septum=, which is drawn out into a
third slender process. These three processes together constitute the
=rostrum= (fig. 6, 2).

Behind the olfactory capsules comes a large, nearly circular, hole,
the =anterior fontanelle=, slightly behind which are the two
=ophthalmic foramina=. The dorsal and ventral boundaries of the orbits
are respectively formed by the prominent =supra-orbital= and
=suborbital ridges=. Behind are the =auditory capsules= (fig. 6, 8),
each of which is marked by a pair of prominent ridges, converging
towards the middle line to a pair of apertures. These apertures
communicate with two canals, the =aqueductus vestibuli=, which lead
into the internal ear. The two ridges lodge respectively the =anterior
and posterior vertical semicircular canals= of the ear.

(_b_) The principal structures to be noted in a side view of the
cranium are contained in the =orbit= or eye-cavity. Near the base of
the orbit at its anterior end is seen the small =orbitonasal foramen=
(fig. 6, 7), for the passage of blood-vessels, not nerves. Above it is
the large =ophthalmic foramen= (fig. 6, 5) so prominent in a dorsal
view of the skull; through it the ophthalmic branches of the fifth and
seventh nerves pass. Slightly further back near the ventral surface is
the large =optic foramen= (fig. 6, II.) for the passage of the second
nerve. Vertically above the optic foramen, near the dorsal surface, is
the very small =foramen for the fourth nerve= (fig. 6, IV.). Behind
and a little above the optic foramen is another small aperture, the
=foramen for the third nerve=. Behind and slightly below this is the
large =foramen for the sixth and main branches of the fifth and
seventh nerves= (fig. 6, V.). In front of and slightly below this
foramen are seen two other small apertures; the more anterior and
ventral of these (fig. 6, 4) is for the passage of a vessel connecting
the efferent artery of the hyoid gill with the internal carotid artery
inside the skull, the more posterior and dorsal is for the
=interorbital canal= (fig. 6, 3) which unites the two orbital sinuses.
Above and very slightly in front of the large foramen for the sixth
and main parts of the fifth and seventh nerves, are two small foramina
(fig. 6, Va., and VIIa.), through which the =ophthalmic branches of
the fifth and seventh nerves= enter the orbit. Behind and slightly
below the large foramen just mentioned is a small hole through which
the external carotid enters the orbit (fig. 6, 9).

[Illustration Fig. 6. LATERAL VIEW OF THE SKULL OF A DOGFISH
(_Scyllium canicula_) × 2/3.]

    1. nasal capsule.
    2. rostrum.
    3. interorbital canal.
    4. foramen for hyoidean artery.
    5. foramen for the exit of the
    ophthalmic branches of
    Vth and VIIth nerves.
    6. foramen through which the
    external carotid leaves the
    orbit.
    7. orbitonasal foramen.
    8. auditory capsule.
    9. foramen through which the
    external carotid enters the
    orbit.
    10. ethmo-palatine ligament.
    11. palato-pterygo-quadrate bar.
    12. Meckel's cartilage.
    13. hyomandibular.
    14. cerato-hyal.
    15. pharyngo-branchial.
    16. epi-branchial.
    17. cerato-branchial.
    18. gill filaments, nearly all have
    been cut off short for the
    sake of clearness.
    19. extra-branchial.
    20. pre-spiracular ligament.
    II. III. IV. V. Va. VIIa. foramina
    for passage of cranial nerves.

Behind the orbit is the =auditory capsule=. This is marked below by a
prominent =surface for the articulation of the hyomandibular=, above
which is the deep =postorbital groove= for the passage of a
blood-vessel, connecting the orbital and anterior cardinal sinuses.

(_c_) Passing to the posterior end of the cranium: in the centre is
seen the large =foramen magnum= through which the brain and spinal
cord communicate. The =notochord= enters the skull just below this
foramen, and on each side of the notochord is a projection, the
=occipital condyle=, by which the first vertebra articulates with the
skull.

External to the condyles are the prominent =pneumogastric foramina=
for the passage of the tenth nerves, and further to the sides, just
beyond the posterior vertical semicircular canals, are a pair of deep
pits in which lie the =foramina for the ninth nerves= (fig. 6, IX).

(_d_) The broad and flat ventral surface of the cranium is continued
in front as the =internasal septum= and terminated laterally by the
=suborbital ridges=. At a little behind the middle it is traversed by
two shallow grooves along which the internal carotid arteries run. At
the divergent ends of these grooves are seen two small apertures
through which the external carotids enter the orbit (fig. 6, 9), and
at the point where they meet is a single small aperture through which
the internal carotid enters the cranium.

(2) THE VISCERAL SKELETON.

The =Visceral skeleton= forms a series of seven cartilaginous arches
or hoops, surrounding the anterior part of the alimentary canal, and
enclosing a wide but rather shallow space.

(_a_) The first or =mandibular arch= is the largest of the series, and
forms the upper and lower jaws. Each half of the upper jaw or
=palato-pterygo-quadrate= bar is formed by a thick cartilaginous rod
which meets its fellow in the middle line in front, the two being
united by ligament. Each half is connected to the cranium just in
front of the orbit by the =ethmo-palatine ligament= (fig. 6, 10), and
at its hind end articulates with one of the halves of the lower jaw.
Each half of the lower jaw or =Meckel's cartilage= (fig. 6, 12) is a
cartilaginous bar, wide behind but narrow in front, where it is united
to its fellow by a median ligament. Imbedded in the tissue external to
the upper jaw are a pair of =labial cartilages=, and a similar but
smaller pair are imbedded in the tissue external to the lower jaw.

The jaws are developed from a structure whose dorsal and ventral
portions subsequently become of very different importance. The ventral
portion forms both upper and lower jaws, the former being developed as
an outgrowth from the latter. The dorsal portion forms only the
=pre-spiracular ligament= (fig. 6, 20), a strong fibrous band
containing a nodule of cartilage, and running from the anterior part
of the auditory capsule to the point where the jaws are connected with
the hyomandibular.

(_b_) The =hyoid arch= consists of a pair of cartilaginous rods which
are attached at their dorsal ends to the cranium, and are united
ventrally by a broad median plate of cartilage, the =basi-hyal=. Each
rod is divided into a dorsal portion, the =hyomandibular= and a
ventral portion, the =cerato-hyal=. The =hyomandibular= (fig. 6, 13)
is a short stout rod of cartilage projecting outwards, and somewhat
backwards and downwards from the cranium, with which it articulates
behind the orbit and below the postorbital groove. Its distal end
articulates with a rather long slender bar, the =cerato-hyal= (fig. 6,
14), which is in its turn attached to the side of the =basi-hyal=. The
=basi-hyal= is a broad plate, rounded in front and drawn out behind
into two processes to which the two halves of the first branchial arch
are attached. The posterior surfaces of both hyomandibular and
cerato-hyal bear slender cartilaginous processes, the =gill rays=. The
hyoid arch forms the main =suspensorium= or means by which the jaws
are attached to the cranium. This attachment is chiefly brought about
by a series of short ligaments which connect the posterior ends of
both upper and lower jaws with the hyomandibular, but there is also a
ligament connecting the lower jaw with the cerato-hyal. The attachment
of the jaws to the cranium is also partially effected by the
pre-spiracular and ethmo-palatine ligaments.

(_c_) Each of the five =branchial arches= is a hoop, incomplete above
and formed of four or more pieces of cartilage. The most dorsal
elements, the =pharyngo-branchials=, are flattened, pointed plates
whose free inner ends run obliquely backwards, and terminate below the
vertebral column. They are connected at their outer ends with the
short broad =epi-branchials= (fig. 6, 16) which lie at the sides of
the pharynx. From the epi-branchials arise the long =cerato
branchials= (fig. 6, 17) which run forwards and inwards along the
ventral wall of the pharynx. The first four cerato-branchials are
connected with small rods, the =hypo-branchials=, which run backwards
to meet one another in the middle line. The last two pairs of
hypo-branchials and the fifth cerato-branchials are connected with a
broad median plate, the =basibranchial=. Along the outer sides of the
second, third and fourth cerato-branchials are found elongated curved
rods, the =extra-branchials= (fig. 6, 19). The epi-branchials and
cerato-branchials bear gill rays along their posterior borders.

C. THE SKELETON OF THE MEDIAN FINS.

The =dorsal fins= have a skeleton consisting of a series of short
cartilaginous rods, the =basals= or basalia, which slope obliquely
backwards. Their bases are imbedded in the muscles of the back, while
their free ends bear a number of small polygonal cartilaginous plates,
the =radials= or radiale. Associated with this cartilaginous skeleton
are a number of long slender horny fibres, the fin-rays, which have
been already referred to in connection with the exoskeleton. The
skeleton of the other median fins mainly consists of these fibres, the
cartilaginous portion being reduced or absent.

2. THE APPENDICULAR SKELETON.

This includes the skeleton of the two pairs of limbs and of their
respective girdles.

THE PECTORAL GIRDLE forms a crescent-shaped hoop of cartilage,
incomplete above and lying just behind the visceral skeleton. The
mid-ventral part of the hoop is the thinnest portion, and is drawn out
in front into a short rounded process which is cupped dorsally and
supports part of the floor of the pericardium (fig. 7, 1). On each
side of this flattened mid-ventral portion the arch becomes very thick
and bears on its outer border a surface with which the three basal
cartilages of the fin articulate. The dorsal ends or scapular portions
of the girdle form a pair of gradually tapering horns.

THE PECTORAL FIN articulates with the pectoral girdle by means of
three basalia or basal cartilages, the =propterygium=, =meso
pterygium= and =meta-pterygium=. The most anterior and the smallest of
these is the =propterygium= (fig. 7, 5), while the most posterior
one, the =meta-pterygium= (fig. 7, 3), is much the largest. Along the
outer borders of the three basalia are arranged a series of close set
cartilaginous pieces, the =radiale=. The propterygium supports only a
single radial, which is however much larger than any of the others.
The meso-pterygium also supports only a single radial which divides
distally.

[Illustration FIG. 7. SEMIDORSAL VIEW OF THE PECTORAL GIRDLE AND FINS
OF A DOGFISH (_Scyllium canicula_) × 2/3.

The gaps between the radiale are blackened.

    1. hollow in the mid-ventral part
    of the pectoral girdle which
    supports the pericardium.
    2. dorsal (scapular portion) of
    pectoral girdle.
    3. meta-pterygium.
    4. meso-pterygium.
    5. propterygium.
    6. propterygial radial.
    7. meso-pterygial radial.
    8. meta-pterygial radial.
    9. outline of the distal part of
    the fin which is supported
    by horny fin-rays.]

The meta-pterygium bears about twelve long narrow radials, the first
nine of which are traversed by a transverse joint at about two-thirds
of the way from their origin. Succeeding the radials are a series of
small polygonal pieces of cartilage arranged in one or more rows and
attached to the ends of the radials, and finally the fin is completed
by the dermal fin-rays.

[Illustration FIG. 8. DORSAL VIEW OF THE PELVIC GIRDLE AND FINS OF A
MALE DOGFISH (_Scyllium canicula_).

    1. pelvic girdle.
    2. basi-pterygium.
    3. clasper.
    4. radiale.]

THE PELVIC GIRDLE is much smaller than the pectoral. It is formed of a
stout nearly straight bar of cartilage placed transversely across the
ventral region of the body. The bar has no dorsal or lateral
extensions, and is terminated by short blunt processes. It bears on
its posterior surface a pair of facets with which the pelvic fins
articulate.

THE PELVIC FIN is smaller and more simply constructed than is the
pectoral. It consists of a long, somewhat curved rod, the
=basi-pterygium= (fig. 8, 2), running directly backwards on the inner
side of the fin, and articulating in front with the pelvic girdle.
From its outer side arise a series of about fourteen parallel
cartilaginous radials which bear smaller polygonal pieces. The
anterior one or two of these radials may articulate independently with
the pelvic girdle. In the adult male dogfish the distal end of the
basi-pterygium bears a stout rod nearly as long as itself, and grooved
on the dorsal surface. This is the skeleton of the =clasper= (fig. 8,
3).

FOOTNOTE:

[34] See Marshall and Hurst's _Practical Zoology_, 4th ed. London,
1895, p. 214.

CHAPTER VII.

THE SKELETON OF THE CODFISH. (_Gadus morrhua._)

I. EXOSKELETON.

The exoskeleton includes

(1) =Scales.= These are of the type known as =cycloid= and consist of
flat rounded plates composed of concentrically arranged laminae of
calcified matter, with the posterior margin entire. The anterior end
of each scale is imbedded in the skin and is overlapped by the
preceding scales.

(2) The =teeth=. These are small, pointed, calcified structures
arranged in large groups on the premaxillae, mandible, vomer, and
superior and inferior pharyngeal bones.

(3) The =fin-rays=. These are delicate, nearly straight bony rods
which support the fins.

II. ENDOSKELETON.

The endoskeleton of the Codfish, though partially cartilaginous, is
mainly ossified.

It is divisible into an =axial portion=, including the skull,
vertebral column, ribs, and skeleton of the median fins, and an
=appendicular portion=, including the skeleton of the paired fins and
their girdles.

1. THE AXIAL SKELETON.

A. THE VERTEBRAL COLUMN.

This consists of a series of some fifty-two vertebrae, all completely
ossified.

It is divisible into two regions only, viz. the =trunk= region, the
vertebrae of which bear movable ribs, and the =caudal= or =tail=
region, the vertebrae of which do not bear movable ribs.

=Trunk vertebrae.=

These are seventeen in number; the ninth may be described as typical
of them all. It consists of a short deeply biconcave =centrum= whose
two cavities communicate by a narrow central canal. From the dorsal
surface of the anterior half of the centrum arise two strong plates,
the dorsal or =neural processes=, which are directed obliquely
backwards and meet forming the dorsal or =neural arch=. This is
produced into a long backwardly-directed dorsal or =neural spine=.

From the lower part of the anterior edge of each neural arch arise a
pair of blunt triangular projections which overhang the posterior half
of the preceding centrum, and bear a pair of flattened surfaces which
correspond to the anterior or =prezygapophyses= of most vertebrae,
they differ however from ordinary prezygapophyses in the fact that
they look downwards and outwards. From the posterior end of the
centrum arise a pair of short blunt processes each of which bears an
upwardly- and inwardly-directed articulating surface corresponding to
a =postzygapophysis=.

The two halves of the ventral arch form a pair of large
=ventri-lateral processes= which arise from the anterior half of the
centrum and pass outwards and slightly backwards and downwards.

Behind these there arises on each vertebra a second outgrowth which is
small and flattened, and like the ventri-lateral process serves to
protect the air-bladder. The surface of the centrum is marked by more
or less wedge-shaped depressions, one in the mid-dorsal line, and two
on the ventral surface immediately mesiad to the bases of the
ventri-lateral process. There are also a number of smaller
depressions.

The space between one centrum and the next is in the fresh skeleton
filled up by the gelatinous remains of the =notochord=.

The first few vertebrae differ from the others in having very short
centra and no ventri-lateral processes.

The first vertebra comes into very close relation to the posterior
part of the skull, articulating with the exoccipitals. In the next few
vertebrae the centra gradually lengthen, and at the fourth or fifth
vertebra the ventri-lateral processes appear and gradually increase in
size as followed back. They likewise gradually come to arise at a
lower level on the centrum, and also become more and more downwardly
directed, till at the last trunk vertebra they nearly meet.

The =neural spines= of the anterior trunk vertebrae are much longer
than those of the posterior ones, that of the first vertebra being the
largest and longest of all, and articulating with the skull. The
spinal nerves pass out through wide notches or spaces between the
successive neural arches.

=Caudal vertebrae.=

The caudal vertebrae are about thirty-five in number, each consists of
a centrum with a slender backwardly-directed dorsal or neural arch,
similar to those of the posterior trunk vertebrae. The two halves of
the ventral or haemal arch however do not form outwardly-directed
ventri-lateral processes, but arise on the ventral surface of the
centrum, and passing downwards meet and enclose a space; they thus
form a complete canal, and are prolonged into a backwardly-directed
ventral or =haemal spine=. The anterior haemal arches are much larger
than the corresponding neural arches, but when followed back they
gradually decrease in size, till at about the twenty-fourth caudal
vertebra they are nearly as small as the neural arches. The last
caudal vertebra is succeeded by a much flattened =hypural= bone or
=urostyle=, which together with the posterior neural and haemal spines
supports the tail-fin.

B. THE RIBS.

The =ribs= are slender, more or less cylindrical bones attached to the
poster-dorsal faces of the ventri-lateral processes of all the trunk
vertebrae except the first and second. The earlier ones are thicker
and more curved; the later ones thinner and more nearly straight. The
ribs are homologous with the distal parts of the haemal arches of the
caudal vertebrae.

Associated with the ribs are a second series of rib-like bones, the
=intermuscular bones=. These are slender, curved bones which arise
from the ribs or from the ventri-lateral processes at a distance of
about an inch from the centra, and curve upwards, outwards and
backwards. In the anterior region where the ventri-lateral processes
are short they arise from the ribs, further back they arise from the
ventri-lateral processes.

C. THE UNPAIRED OR MEDIAN FINS.

These are six in number, three being =dorsal=, one =caudal= and two
=anal=.

The =dorsal= and =anal= fins each consist of two sets of structures,
the =fin-rays= and the =interspinous bones=. Each fin-ray forms a
delicate, nearly straight, bony rod which becomes thickened and
bifurcated at its proximal or vertebral end, while distally it is
transversely jointed and flexible, frequently also becoming more or
less flattened.

The first dorsal fin has thirteen rays, the second, sixteen to
nineteen, the third, seventeen to nineteen. The first anal fin has
about twenty-two, the second anal fourteen. In each fin the posterior
rays rapidly decrease in size when followed back.

The =interspinous bones= of the dorsal and anal fins alternate with
the neural and haemal spines respectively, and form short,
forwardly-projecting bones, each attached proximally to the base of
the corresponding fin-ray.

The =caudal fin= consists of a series of about forty-three rays which
radiate from the posterior end of the vertebral column, being
connected with the urostyle or hypural bone, and with the posterior
neural and haemal spines without the intervention of interspinous
bones. Like the other fin-rays those forming the caudal fin are
transversely jointed, and are widened and frayed out distally.

The tail-fin in the Cod is =homocercal=, i.e. it appears to be
symmetrically developed round the posterior end of the vertebral
column, though in reality a much greater proportion is attached below
the end of the vertebral column than above it. It is a masked
heterocercal tail.

THE SKULL.

Owing to the fact that very little cartilage remains in the skull of
the adult Codfish, its relation to the completely cartilaginous skull
of the Dogfish is not easily seen. Before describing it therefore, the
skull of the Salmon will be described, as it forms an intermediate
type.

THE SKULL OF THE SALMON[36].

The Salmon's skull consists of (1) the =chondrocranium=, which remains
partly cartilaginous and is partly converted into cartilage bone,
especially in the occipital region, (2) a large series of plate-like
membrane bones.

THE CHONDROCRANIUM.

This is an elongated structure, wide behind owing to the fusion of the
large auditory capsules with the cranium, and elongated and tapering
considerably in front; in the middle it is much contracted by the
large orbital cavities.

DORSAL SURFACE OF THE CRANIUM.

In the centre of the posterior end of the dorsal surface is the
=supra-occipital= (fig. 9, A, 1) with a prominent posterior ridge.
It is separated by two tracts of unossified cartilage from the large
series of bones connected with the =auditory organ=. The first of
these is the =epi-otic= (fig. 9, 2), which is separated by only a
narrow tract of cartilage from the supra-occipital, and is continuous
laterally with the large =pterotic= (fig. 9, A, 3) which overlaps in
front a smaller bone, the =sphenotic= (fig. 9, 4). Both epi-otic and
pterotic are drawn out into rather prominent backwardly-projecting
processes.

[Illustration FIG. 9. A. DORSAL AND B. VENTRAL VIEW OF THE CRANIUM OF
A SALMON (_Salmo salar_) from which most of the membrane bones have
been removed (after PARKER). Cartilage is dotted.

    1. supra-occipital.
    2. epi-otic.
    3. pterotic.
    4. sphenotic.
    5. frontal.
    6. median ethmoid.
    7. parietal.
    8. lateral ethmoid.
    9. parasphenoid.
    10. vomer.
    11. exoccipital.
    12. opisthotic.
    13. alisphenoid.
    14. orbitosphenoid.
    16. foramen for passage of an artery.
    17. pro-otic.
    18. articular surface for hyomandibular.

II. VII. IX. X. foramina for the passage of cranial nerves.]

The greater part of the remainder of the dorsal surface is formed of
unossified cartilage which is pierced by three large vacuities or
=fontanelles=. The anterior fontanelle is unpaired, and lies far
forward near the anterior end of the long cartilaginous snout, the two
larger posterior ones lie just in front of the supra-occipital and
lead into the cranial cavity. In front of the orbit the skull widens
again, and is marked by two considerable =lateral ethmoid= (fig. 9, 8)
ossifications. In front of these are a pair of deep pits, the =nasal
fossae=, at the base of which are a pair of foramina through which the
olfactory nerves pass out; they communicate with a space, the =middle
narial cavity=, seen in a longitudinal section of the skull.

The long cartilaginous snout is more or less bifid in front,
especially in the male (fig. 9).

POSTERIOR END OF THE CRANIUM.

The =foramen magnum= forms a large round hole leading into the cranial
cavity, and is bounded laterally by the two =exoccipitals= and below
by them, and to a very slight extent by the =basi-occipital=, the
three bones together forming a concave =occipital condyle= by which
the vertebral column articulates with the skull.

The exoccipitals are connected laterally with a fourth pair of
auditory bones, the =opisthotics=, and just meet the epi-otics
dorsolaterally, while dorsally they are separated by a wide tract of
unossified cartilage from the supra-occipital.

The opisthotics are connected laterally with the pterotics.

SIDE OF THE CRANIUM.

At the posterior end is seen the =basi-occipital= in contact above
with the =exoccipital=, which is pierced by a prominent foramen for
the exit of the tenth nerve. In front of this lies a small foramen,
sometimes double, for the ninth nerve.

[Illustration FIG. 10. LATERAL VIEW OF THE CHONDROCRANIUM OF A SALMON
(_Salmo salar_) (after PARKER). A few membrane bones are also shown.
Cartilage is dotted.

    1. supra-occipital.
    2. epi-otic.
    3. pterotic.
    4. opisthotic.
    5. exoccipital.
    6. basi-occipital.
    7. parasphenoid.
    8. sphenotic.
    9. alisphenoid.
    10. orbitosphenoid.
    11. lateral- or ectethmoid.
    12. olfactory pit; the vomerine teeth are seen just below.
    14. pro-otic.
    15. basisphenoid.
    16. foramen for the passage of an artery.
    17. anterior fontanelle.
    18. posterior fontanelle.

I. II. V. VII. IX. X. foramina for the passage of cranial nerves.]

In front of the exoccipital is the large =pro-otic= pierced by two
prominent foramina. Through the more dorsal of these (fig. 10, VII.)
the facial nerve passes out, while the more ventral (fig. 10, 16) is
for the passage of an artery. Dorsal to the exoccipital are the
=opisthotic= and =pterotic=, and dorsal to the pro-otic is the
=sphenotic=. The =pterotic= is marked by a prominent groove often
lined by cartilage, which is continued forwards along a tract of
cartilage between the pro-otic and sphenotic. With this groove the
hyomandibular articulates.

There are considerable ossifications in the sphenoidal region of the
side of the cranium. The anterior boundary of the posterior fontanelle
is formed by the large =alisphenoid=, which is continuous behind with
the pro-otic and sphenotic, and below with a slender =basisphenoid=.
Both in front of and behind the basisphenoid there are considerable
vacuities in the walls of the cranium; through the posterior of these
openings (fig. 10, V.) the main part of the trigeminal nerve passes
out, and through the anterior one, the optic (fig. 10, II.). The
alisphenoid is continuous in front with the =orbitosphenoid= (fig. 10,
10), which is pierced by the foramen for the exit of the first nerve
(fig. 10, I.), and in front of the orbitosphenoid there is a large
vacuity. The =lateral ethmoid= is seen in the side view as well as in
the dorsal view. Further forwards are seen the olfactory pits, and the
long cartilaginous snout.

A =ventral view= of the cartilaginous cranium shows much the same
points as the side view. The basisphenoid appears on the surface
immediately in front of the basi-occipital.

THE SKULL WITH MEMBRANE BONES.

The =dorsal surface=. The greater part of the dorsal surface in front
of the supra-occipital is overlaid by a pair of large rough _frontals_
(figs. 9, A, 5, and 10, 5). They cover the posterior fontanelles and
stretch over from the sphenotic to the lateral ethmoid, forming a roof
for the orbit. They meet in the middle line behind, but in front are
separated by a narrow tract of unossified cartilage, and are
overlapped by the _median ethmoid_ (figs. 9, A, 6, and 11, 6). At the
sides of the supra-occipital behind the frontals are a pair of small
_parietals_ (figs. 9, A, 7, and 11, 7).

[Illustration FIG. 11. LATERAL VIEW OF THE SKULL OF A SALMON (_Salmo
salar_) (after PARKER). Cartilage is dotted.

    1. supra-occipital.
    2. epi-otic.
    3. pterotic.
    4. sphenotic.
    5. frontal.
    6. median ethmoid.
    7. parietal.
    8. nasal.
    9. lachrymal.
    10. suborbital.
    11. supra-orbital.
    12. cartilaginous sclerotic.
    13. ossification in sclerotic.
    14. meso-pterygoid.
    15. meta-pterygoid.
    16. palatine.
    17. jugal.
    18. quadrate.
    19. maxillae
    20. premaxillae.
    21. articular.
    22. angular.
    23. dentary.
    24. hyomandibular.
    25. symplectic.
    26. epi-hyal.
    27. cerato-hyal.
    28. hypo-hyal.
    29. glosso-hyal.
    30. opercular.
    31. sub-opercular.
    32. infra-opercular.
    33. pre-opercular.
    34. supratemporal.
    35. branchiostegal rays.
    36. basi-branchiostegal.]

In a =ventral view= the cranium is seen to be chiefly covered by two
large membrane bones, the _parasphenoid_ (fig. 9, B, 9) behind, the
_vomer_ in front. A view of the =posterior end= differs from that of
the cartilaginous cranium only in the fact that the end of the
_parasphenoid_ appears lying ventral to the basi-occipital.

The =lateral view= differs very markedly from that of the
cartilaginous cranium, there being a great development of membrane
bone in connection with the jaws and branchial apparatus. Lying
dorsally are seen the _median ethmoid_, _frontal_, _parietal_, and
=supra-occipital= as before. Lying external to the middle of the
_median ethmoid_ is seen the small _nasal_ (fig. 11, 8), and below the
hinder part is the _lachrymal_. The _lachrymal_ (fig. 11, 9) forms the
first of a series of seven small bones which surround the orbit
forming the =orbital ring=. Of these the one lying immediately in the
mid-ventral line of the orbit is the _suborbital_, while the one lying
in the mid-dorsal line and attached to the frontal is the
_supra-orbital_ (fig. 11, 11). The orbit has a cartilaginous
_sclerotic_ in which are two small ossifications (fig. 11, 13)
laterally placed.

BONES OF THE UPPER JAW.

The =palato-pterygo-quadrate bar= is in a very different condition
from that of the dogfish, it is partially cartilaginous, partially
converted into cartilage bone, partially overlapped by membrane bone.
It is narrow in front but becomes much broader and deeper when
followed back. Its anterior end forms the =palatine= which bears
teeth, and in front is completely ossified, while behind the cartilage
is only sheathed by bone.

Just behind the palatine the outer part of the cartilage is ossified,
forming two small bones, the =pterygoid= and =meso-pterygoid=, while
behind them is a larger, somewhat square bone, the =meta-pterygoid=
(fig. 11, 15).

Below the meta-pterygoid is a tract of unossified cartilage, and then
comes the =quadrate= (fig. 11, 18).

The lower angle of the quadrate bears a cartilaginous =condyle= with
which the mandible articulates. In front of the palatine the
cartilaginous snout is overlapped by three membrane bones, the
_jugal_, _maxilla_ and _premaxillae_.

The _premaxillae_ (fig. 11, 20), the largest of these, overlaps the
maxilla behind; both bones bear teeth. The _jugal_ (fig. 11, 17) lies
above the maxilla and overlaps it in front.

THE LOWER JAW.

The =lower jaw= is a strong bar and is like the upper jaw, partly
cartilaginous, forming =Meckel's cartilage=, partly ossified, and
sheathed to a considerable extent in membrane bone.

The outer side and posterior end is ossified, forming the large
=articular= (fig. 11, 21), but the condyle is cartilaginous and the
anterior part of the articular forms merely a splint on the outer side
of Meckel's cartilage, which extends beyond it for a considerable
distance. The angle of the jaw just below the condyle is formed by a
small _angular_ (fig. 11, 22), and the anterior two-thirds of the jaw
is sheathed in the large tooth-bearing _dentary_ (fig. 11, 23).

THE HYOID ARCH.

The =hyoid arch= has a number of ossifications in it and is closely
connected with the mandibular arch.

The =hyomandibular= (fig. 11, 24) is a large bone which articulates
with a shallow groove lined by cartilage and formed partly in the
pterotic, partly in front of it. The hyomandibular is overlapped in
front by the meta-pterygoid, while below it tapers and is succeeded by
a small area of unossified cartilage followed by the forwardly
directed =symplectic= which fits into a groove in the quadrate.

The unossified tract between the hyomandibular and symplectic is
continuous in front with a strong bar, which remains partly
cartilaginous and is partly converted into cartilage bone. The
proximal part is ossified, forming the =epi-hyal=, the middle part
forms the =cerato-hyal= (fig. 11, 27), in front of which is the small
=hypo-hyal=. The hyoid arches of the two sides are united by the large
tooth-bearing =glosso-hyal= (fig. 11, 29). Attached to the lower
surface of the hyoid arch are a series of twelve flat _branchiostegal
rays_ (fig. 11, 35). Each overlaps the one in front of it, the
posterior one being the largest. The branchiostegal rays of the two
sides are united in front by an unpaired membrane bone, the
_basi-branchiostegal_ (fig. 11, 36).

=Opercular bones.= Behind the hyomandibular there is a large bony
plate, the =operculum=, formed of four large membrane bones. The
anterior of these, the _pre-opercular_ (fig. 11, 33), is crescentic in
shape, and with its upper end a small _supratemporal_ (fig. 11, 34) is
connected.

Behind the upper part of the pre-opercular is the largest of the
opercular bones, the _opercular proper_. Its lower edge overlaps the
sub-opercular, and both opercular and sub-opercular are overlapped by
the _infra-opercular_ (fig. 11, 32) in front. The infra-opercular is
in its turn overlapped by the _pre-opercular_.

BRANCHIAL ARCHES.

There are five branchial arches, the first four of which bear gill
rays. Each of the first three consists of a shorter upper portion
directed obliquely backwards and outwards, and a longer lower portion
forming a right angle with the upper and directed obliquely forwards
and inwards. The greater part of each arch is ossified.

The upper part of either of the first two consists of a short tapering
=pharyngo-branchial= directed inwards, and of a long =epi-branchial=
tipped with cartilage at both ends. The junction of the upper and
lower parts is formed by a cartilaginous hinge-joint between the
epi-branchial and cerato-branchial. The =cerato-branchial= is a long
bony rod separated by a short area of cartilage from the
=hypo-branchial=, which is succeeded by the =basibranchial= meeting
its fellow in the middle line. The =fourth arch= has a short
epi-branchial and no ossified pharyngo-branchial, while the fifth is
reduced to little more than the cerato-branchial, which bears a few
teeth on its inner edge. All the branchial arches have projecting from
their surfaces a number of little processes which act as strainers.
The first and fourth arches have one series of these, the second and
third have two.

THE SKULL OF THE CODFISH[37].

A full description having been already given of the Salmon's skull,
that of the Codfish will be described in a briefer manner. The skull
is very fully ossified, and the great number of plate-like bones
render it a very complicated structure.

THE CRANIUM.

At the posterior end of the dorsal surface is the large
=supra-occipital=, which is drawn out behind into the large blade-like
=occipital spine=. On each side of the supra-occipital are the small
irregular _parietals_, while in front of it the roof of the skull is
mainly formed by the very large unpaired _frontal_.

A complicated series of bones are developed in connection with the
=auditory capsule=, which forms a large projecting mass united with
the side of the cranium and drawn out behind into a pair of strong
processes, the =epi-otic= and =parotic= processes. Both these
processes are connected behind with a large V-shaped bone, the
_post-temporal_ (fig. 13, 1), which will be described when dealing
with the pectoral girdle. The =epi-otic process= is formed by the
=epi-otic=, which is continuous in front with the parietal. The
=parotic process= is formed by two larger bones, a more dorsal one,
the =pterotic=, and a more ventral and internal one, the =opisthotic=,
which is continuous in front with the large =pro-otic=. Intervening
between the pterotic and frontal is another rather large bone, the
=sphenotic=, this articulates below with the pro-otic. The pterotic
and sphenotic together give rise to a large concave surface by which
the hyomandibular articulates with the cranium. Several of the cranial
nerves pass out through the bones of the auditory capsule. The ninth
leaves by a foramen near the posterior border of the opisthotic, the
fifth and seventh by a notch in the anterior border of the pro-otic.

A number of bones are likewise developed in connection with the orbit
forming the =orbital ring=. Of these the most anterior, the
_lachrymal_, is much the largest, the others are five to seven in
number, the most ventral being the _suborbital_. The sclerotic coat of
the eye is cartilaginous.

Two pairs of bones and one unpaired bone are developed in connection
with the =olfactory capsules=, of these, the _nasals_ are narrow bones
lying next the lachrymals, but nearer the middle line; they overlap
the second pair of bones, the irregular =lateral ethmoids=. These meet
one another in the middle line, and are overlapped behind by the
frontal. They articulate laterally with the lachrymal and palatine,
and ventrally with the parasphenoid.

In a =posterior view= the foramen magnum and the four bones which
surround it and together form the occipital segment are well seen. On
the ventral side is the =basi-occipital=, terminated posteriorly by a
slightly concave surface which articulates with the centrum of the
first vertebra. The sides of the foramen magnum are formed by the
=exoccipitals=, a pair of very irregular bones, pierced by a pair of
prominent foramina for the exit of the tenth nerves. The exoccipitals
also bear a pair of surfaces for articulation with corresponding ones
on the neural arch of the first vertebra. The most dorsal of the four
bones is the supra-occipital.

On the ventral surface of the cranium in front of the basi-occipital
is seen the _parasphenoid_, a very long narrow bone which underlies
the greater part of the cranium. Behind, it articulates dorsally with
the basi-occipital and dorsolaterally with the pro-otics and
opisthotics, in front it articulates dorsally with the lateral ethmoid
and ventrally with the vomer. At the sides of the parasphenoid are the
small =alisphenoids= articulating above with the postfrontals, in
front with the frontals, and behind with the pro-otics.

The _vomer_ is an unpaired bone lying immediately in front of the
parasphenoid. In front it terminates with a thickened curved margin
bearing several rows of small teeth; behind it tapers out into a long
process which underlies the anterior part of the parasphenoid.
Immediately dorsal to the vomer is another median bone, the _median
ethmoid_; this is truncated in front and tapers out behind into a
process which fits into a groove on the ventral side of the frontal.

BONES IN CONNECTION WITH THE UPPER JAW.

These bear a close resemblance to those of the Salmon. The most
anterior bone is the _premaxillae_, a thick curved bone meeting its
fellow in the middle line. The point of junction of the two is drawn
out into a short process, and the oral surface is thickly covered with
small teeth. The dorsal ends of the premaxillae are seen in the fresh
skull to meet a large patch of cartilage. Behind the premaxillae is the
_maxilla_, a long rod-like toothless bone, somewhat expanded at the
upper end where it articulates with the premaxillae and vomer.

Articulating in front with the anterior end of the maxilla and with
the =lateral ethmoid= is a very irregular bone, the =palatine= (fig.
12, 1); it articulates behind with two flat bones, the =pterygoid= and
=meso-pterygoid=. The pterygoid is united behind with two more bones,
the =quadrate= (fig. 12, 4) and =meta-pterygoid=. The =quadrate= is a
rather stout irregular bone, bearing on its lower surface a prominent
saddle-shaped articulating surface for the mandible. The palatine,
pterygoid and quadrate bones are the ossified representatives of the
palato-pterygo-quadrate bar of the Dogfish.

[Illustration FIG. 12. MANDIBULAR AND HYOID ARCHES OF A COD (_Gadus
morrhua_) × 1/2 (Brit. Mus.).

    1. palatine.
    2. meso-pterygoid.
    3. pterygoid.
    4. quadrate.
    5. symplectic.
    6. meta-pterygoid.
    7. hyomandibular.
    8. angular.
    9. articular.
    10. dentary.
    11. inter-hyal.
    12. epi-hyal.
    13. cerato-hyal.
    14. hypo-hyal.
    15. uro-hyal.
    16. branchiostegal rays.]

The quadrate is united behind with the =symplectic= (fig. 12, 5), and
the meta-pterygoid with the symplectic and =hyomandibular=, both of
which bones will be described immediately in connection with the hyoid
arch.

THE LOWER JAW.

The =lower jaw or mandible= like that of the Salmon is partly
cartilaginous, forming =Meckel's cartilage=, partly formed of
cartilage bone, partly of membrane bone. Meckel's cartilage is of
course not seen in the dried skull.

The lower jaw includes one cartilage bone, the =articular= (fig. 12,
9), this is a large bone connected by a saddle-shaped surface with the
quadrate. Meckel's cartilage lies in a groove on its under surface,
and projects beyond it in front. The _angular_ is a small thick bone
united to the lower surface of the articular at its posterior end. The
_dentary_ (fig. 12, 10) is a large tooth-bearing bone meeting its
fellow in the middle line in front, while the articular fits into a
deep notch at its posterior end.

THE HYOID ARCH.

The =hyomandibular= (fig. 12, 7) is a large irregular bone,
articulating by a prominent rounded head with the sphenotic and
pterotic. It is united in front with the meta-pterygoid and
symplectic, and sends off behind a strong process which articulates
with the opercular. The =symplectic= is a long somewhat triangular
bone drawn out in front into a process which fits into a groove on the
inner surface of the quadrate. The distal portion of the hyoid arch is
strongly developed and consists of first the =inter-hyal= (fig. 12,
11), a short bony rod, which articulates dorsally with a patch of
cartilage intervening between the posterior part of the hyomandibular
and the symplectic. Below it is united with the apex of the triangular
=epi-hyal=, a bone suturally connected with the large =cerato-hyal=
(fig. 12, 13) which unites distally with two small =hypo-hyals=. To
the cerato-hyal are attached a series of seven strong curved
cylindrical rods, the _branchiostegal rays_. The first of these is the
smallest and they increase in size up to the last. The four dorsal
ones are attached to the outer surface of the cerato-hyal, the three
ventral ones to its inner surface. Interposed between the hypo-hyals
of the two sides is an unpaired somewhat triangular plate, the
uro-hyal or _basi-branchiostegal_ (fig. 12, 15).

THE BRANCHIAL ARCHES.

The =branchial arches= are five in number and consist of the following
parts on each side. The dorsal end is formed of the =supra-pharyngeal=
bone, a large irregular bone covered ventrally with teeth of a fair
size, and representing the fused =pharyngo-branchials= of the four
anterior arches. Its external surface is continuous with four small
=epi-branchials= which pass horizontally backwards and outwards. Their
distal ends meet four long =cerato-branchials= which are directed
forwards and inwards and form the principal part of the arches.

Each of the first three cerato-branchials articulates ventrally with a
=hypo-branchial=, and the hypo-branchials of the two sides are united
in the middle line by an unpaired =basibranchial=. The third
hypo-branchial is much flattened. The fourth cerato-branchial is
united by cartilage with the posterior surface of the third
hypo-branchial, which it meets near the middle line.

The fifth arch consists only of the cerato-branchial, a wide structure
covered with teeth and generally called the =inferior pharyngeal
bone=.

The skeleton of the =operculum= consists of the same four bones as in
the Salmon, namely the _opercular_, the _infra-opercular_, the
_pre-opercular_ and the _sub-opercular_. Of these the anterior bone,
the _pre-opercular_, is the largest, while the _infra-opercular_ is
the smallest. The _opercular_ has a facet for articulation with the
hyomandibular.

2. THE APPENDICULAR SKELETON.

THE PECTORAL GIRDLE.

This is of a highly specialised type. Membrane bones are greatly
developed, and the cartilage bones, the =scapula= and =coracoid=, are
much reduced in size and importance.

[Illustration FIG. 13. THE RIGHT HALF OF THE PECTORAL GIRDLE AND RIGHT
PECTORAL FIN OF A COD (_Gadus morrhua_) × 1/2 (Brit. Mus.).

    1. post-temporal.
    2. supra-clavicle.
    3. clavicle.
    4. coracoid.
    5. scapula.
    6. post-clavicle.
    7. brachial ossicles.
    8. dermal fin-rays.]

The largest bone in the shoulder girdle is the _clavicle_ (fig. 13,
3), which is irregularly crescent shaped, thick in front and tapering
off behind. To the outer side of its upper part is attached a thick
cylindrical bone, the _supra-clavicle_, which passes upwards and is
connected with a strong =V= shaped bone, the _post-temporal_. The apex
of the =V= meets the supra-clavicle, the inner limb articulates with
the epi-otic process, the outer with the parotic process. Projecting
downwards from the upper part of the clavicle is a long bony rod,
flattened proximally and cylindrical and pointed distally, this is
the _post-clavicle_ (fig. 13, 6).

The =scapula= (fig. 13, 5) is a small irregular plate of bone attached
to the inner side of the middle of the _clavicle_. The =coracoid=[38]
is a larger plate of similar character, irregularly triangular in
shape, attached to the inner side of the clavicle immediately below
the scapula. The scapula and coracoid bear the pectoral fin.

THE PECTORAL FINS.

Each of these consists of four small irregular bones, the =brachial
ossicles= (fig. 13, 7), bearing a series of about nineteen dermal
_fin-rays_. The brachial ossicles represent the reduced and modified
radiale and basalia of cartilaginous fish such as the dogfish. The
fin-rays (fig. 13, 8) which form the whole external portion of the fin
are long slender rods having essentially the same character as those
of the unpaired fins.

THE PELVIC GIRDLE.

The =pelvic girdle= in the Cod as in other Teleosteans is entirely
absent, its place being taken by the enlarged basi-pterygia of the
fins.

THE PELVIC FINS.

These have a very anomalous position in the Cod, being attached to the
throat in front of the pectoral girdle. Each consists of a basal
portion, the =basi-pterygium=, and of a number of dermal rays. The
basi-pterygium consists of an expanded ventral portion which meets its
fellow below in the middle line, and to which the rays are attached,
and of an inwardly-directed dorsal portion which also meets its fellow
and is imbedded in the flesh. The rays are six in number and are long
slender structures similar to those of the other fins.

FOOTNOTES:

[35] See T.J. Parker's _Zootomy_, London, 1884, p. 86.

[36] See W.K. Parker and G.T. Bettany, _The Morphology of the Skull_,
London, 1877, chap. 3.

[37] T.J. Parker, _Zootomy_, London, 1884, p. 91.

[38] According to G. Swirski, _Schultergurtel des Hechtes_, Dorpat,
1880, the true coracoid is aborted, and the so-called coracoid of
Teleosteans is really the precoracoid.

CHAPTER VIII.

GENERAL ACCOUNT OF THE SKELETON IN FISHES[39].

EXOSKELETON.

The most primitive type of exoskeleton is that found in Elasmobranchs
and formed of =placoid= scales; these are tooth-like structures
consisting of dentine and bone capped with enamel, and have been
already described (p. 4). In most Elasmobranchs they are small and
their distribution is fairly uniform, but in the Thornback skate,
_Raia clavata_, they have the form of larger, more scattered spines.
In adult Holocephali and in _Polyodon_ and _Torpedo_ there is no
exoskeleton, in young Holocephali, however, there are a few small
dorsal ossifications.

The plates or scales of many Ganoids may have been formed by the
gradual fusion of elements similar to these placoid scales, and often
bear a number of little tooth-like processes. In _Lepidosteus_,
_Polypterus_, and many extinct species, these _ganoid_ scales, which
are rhomboidal in form and united to one another by a peg and socket
articulation, enclose the body in a complete armour. In _Trissolepis_
part of the tail is covered by rhomboidal scales, while rounded scales
cover the trunk and remainder of the tail. _Acipenser_ and
_Scaphirhynchus_ have large dermal bony plates which are not
rhomboidal in shape and do not cover the whole body. In _Acipenser_ a
single row extends along the middle of the back and two along each
side.

The majority of Teleosteans have thin flattened scales which differ
from those of Ganoids in being entirely mesodermal in origin,
containing no enamel. There are two principal types of Teleostean
scales, the cycloid and ctenoid. A =cycloid= scale is a flat thin
scale with concentric markings and an entire posterior margin. A
=ctenoid= scale differs in having its posterior margin pectinate. The
Dipnoi have overlapping cycloid scales. The rounded scales of _Amia_
and of many fossil ganoids such as _Holoptychius_ are shaped like
cycloid scales, but differ from them in being more or less coated with
enamel. In Eels and some other Teleosteans the scales are completely
degenerate and have almost disappeared. Some Teleosteans, like _Diodon
hystrix_, have scales with triradiate roots from which arise long
sharp spines directed backwards. These scales, which resemble teeth,
contain no enamel; they become erect when the fish inflates its body
into a globular form. Many Siluroids have dermal armour in the form of
large bony plates which are confined to the anterior part of the body.
In _Ostracion_ the whole body is covered by hexagonal plates, closely
united together.

The =fin-rays= are structures of dermal origin which entirely or
partially support the unpaired fins, and assist the bony or
cartilaginous endoskeleton in the support of the paired fins.

In Elasmobranchs, Dipnoi, and Chondrosteous ganoids the skeletons of
the fins are, as a rule, about half of exoskeletal, half of
endoskeletal origin, the proximal and inner portion being
cartilaginous and endoskeletal, the distal and outer portion being
exoskeletal, and consisting of horny or of more or less calcified
fin-rays. In bony Ganoids and Teleosteans the endoskeletal parts are
greatly reduced and the fins come to consist mainly of the fin-rays,
which are ossified and frequently become flattened at their distal
ends.

The fin-rays of the ventral part of the caudal fin are carried by the
haemal arches; those of the dorsal and anal fins and of the dorsal
part of the caudal fin generally by interspinous bones, which in adult
Teleosteans alternate with the neural and haemal spines. In Dipnoi
these interspinous bones articulate with the neural and haemal spines.
In many Siluroids the anterior rays of the dorsal and pectoral fins
are developed into large spines which often articulate with the
endoskeleton, or are sometimes fused with the dermal armour plates.
Similar spines may occur in Ganoids in front of both the dorsal and
anal fins. _Polypterus_ has a small spine or _fulcrum_ in front of
each segment of the dorsal fin. Such spines are often found
fossilised, and are known as _ichthyodorulites_.

Similar spines are found in many Elasmobranchs, but they are simply
inserted in the flesh, not articulated to the endoskeleton. They also
differ from the spines of Teleosteans and Ganoids in the fact that
they are covered with enamel, and often have their edges serrated like
teeth. In the extinct Acanthodii they generally occur in front of all
the fins, paired and unpaired.

In _Trygon_, the Sting-ray, the tail bears a serrated spine which is
used for purposes of offence and defence. Many ichthyodorulites may
have been spines of this nature fixed to the tail, rather than spines
situated in front of the fins. The spines, which are always found in
front of the dorsal fin in Holocephali, agree with those of
Elasmobranchs in containing enamel, and with those of Teleosteans in
being articulated to the endoskeleton.

TEETH.

The teeth of fish[40] are subject to a very large amount of variation,
perhaps to more variation than are those of any other class of
animals. Sometimes, as in adult Sturgeons, they are entirely absent,
sometimes they are found on all the bones of the mouth, and also on
the hyoid and branchial arches. The teeth are all originally developed
in the mucous membrane of the mouth, but they afterwards generally
become attached to firmer structures, especially to the jaws. In
Elasmobranchs, however, they are generally simply imbedded in the
tough fibrous integument of the mouth. Their attachment to the jaws
may take place in three different ways.

[Illustration FIG. 14. DIAGRAM OF A SECTION THROUGH THE JAW OF A SHARK
(_Odontaspis americanus_) showing the succession of teeth (Brit. Mus.
from specimen and diagram).

    1. teeth in use.
    2. teeth in reserve.
    3. skin.
    4. cartilage of the jaw.
    5. encrusting calcification of cartilage.
    6. connective tissue.
    7. mucous membrane of the mouth.]

(1) By an elastic hinge-joint, as in the Angler (_Lophius_), and the
Pike (_Esox lucius_). In the Angler the tooth is held by a fibrous
band attaching its posterior end to the subjacent bone, in the Pike by
uncalcified elastic rods in the pulp cavity.

(2) By ankylosis, i.e. by the complete union of the calcified tooth
substance with the subjacent bone. This is the commonest method among
fish.

(3) By implantation in sockets. This method is not very common among
fish. The teeth are sometimes, as in _Lepidosteus_, ankylosed to the
base of the socket. In this genus there is along each ramus of the
mandible a median row of large teeth placed in perfect sockets, and
two irregular lateral rows of small teeth ankylosed to the jaw.

Dentine, enamel and cement are all represented in the teeth of fishes,
but the enamel is generally very thin, and cement is but rarely
developed. Dentine forms the main bulk of the teeth; it is sometimes
of the normal type, but generally differs from that in higher
vertebrates in being vascular, and is known as _vasodentine_. A third
type occurs, known as _osteodentine_; it is traversed by canals
occupied by marrow, and is closely allied to bone.

[Illustration FIG. 15. PART OF THE LOWER JAW OF A SHARK (_Galeus_)
(from OWEN after ANDRÉ).

1. teeth in use.

2. reserve teeth folded back.

3. part of the caudal spine of a Sting-ray (_Trygon_) which has
pierced the jaw and affected the growth of the teeth.]

The teeth are generally continually renewed throughout life, but
sometimes one set persists.

The teeth of Selachii are fundamentally identical with placoid
scales. They are developed from a layer of dental germs which occurs
all over the surface of the skin, except in the region of the lips. At
this point the layer of tooth-producing germs extends back into the
mouth, being projected by a fold of the mucous membrane (fig. 14, 7).
Here new teeth are successively formed, and as they grow each is
gradually brought into a position to take the place of its predecessor
by the shifting outwards of the gum over the jaw. Owing to this
arrangement sharks have practically an unlimited supply of teeth
(figs. 14 and 15).

Two principal types of teeth are found in ELASMOBRANCHS. In Sharks and
Dogfish, on the one hand, the teeth are very numerous, simple, and
sharp-pointed, and are with or without serrations and lateral cusps.
Many Rays and fossil Elasmobranchs, on the other hand, have broad
flattened teeth adapted for crushing shells. Intermediate conditions
occur between these two extremes. Thus in _Cestracion_ and many
extinct sharks, such as _Acrodus_, while the median teeth are sharp,
the lateral teeth are more or less flattened and adapted for crushing.
In various species belonging to the genus _Raia_ the teeth of the male
are sharp, while those of the female are blunt. A very specialised
dentition is met with in the Eagle-rays (Myliobatidae), in which the
jaws are armed with flattened angular tooth-plates, arranged in seven
rows, forming a compact pavement; the plates of the middle row are
very wide and rectangular, those of the other rows are much smaller
and hexagonal. Lastly, in _Cochliodus_ the individual crushing teeth
are fused, forming two pairs of spirally-coiled dental plates on each
side of each jaw. _Pristis_, the Saw-fish, has a long flat
cartilaginous snout, bearing a double row of persistently-growing
teeth planted in sockets along its sides. Each tooth consists of a
number of parallel dentinal columns, united at the base, but elsewhere
distinct.

In the HOLOCEPHALI--_Chimaera_, _Hariotta_ and _Callorhynchus_--only
three pairs of teeth or dental plates occur, two pairs in the upper
jaw, one in the lower. These structures persist throughout life and
grow continuously. The upper tooth structures are attached
respectively to the ethmoid or vomerine region of the skull, and to
the palato-pterygoids. The vomerine teeth are small, while those
attached to the mandible and the palato-pterygoid region are large and
bear several roughened ridges adapted for grinding food. The teeth of
the two opposite sides of the jaw meet in a median symphysis. The
teeth of _Chimaera_ are more adapted for cutting, those of
_Callorhynchus_ for crushing. Many extinct forms are known, some of
whose teeth are intermediate in structure between those of _Chimaera_
and _Callorhynchus_.

The teeth of GANOIDS are also extremely variable. Among living forms,
the Holostei are more richly provided with teeth than are any other
fishes, as they may occur on the premaxillae, maxillae, palatines,
pterygoids, parasphenoid, vomers, dentaries, and splenials. Among the
Chondrostei, on the other hand, the adult Acipenseridae are toothless;
small teeth however occur in the larval sturgeon, and in _Polyodon_
many small teeth are found attached merely to the mucous membrane of
the jaws. Many fossil Ganoids have numerous flattened or knob-like
teeth, borne on the maxillae, palatines, vomers and dentaries. Others
have a distinctly heterodont dentition. Thus in _Lepidotus_ the
premaxillae bear chisel-like teeth, while knob-like teeth occur on the
maxillae, palatines and vomers. In _Rhizodus_ all the teeth are
pointed, but while the majority are small a few very large ones are
interspersed.

In TELEOSTEANS, too, the teeth are eminently variable both in form and
mode of arrangement. They may be simple and isolated, or compound, and
may be borne on almost any of the bones bounding the mouth cavity, and
also as in the Pike, on the hyoid and branchial arches. The splenial
however never bears teeth and the pterygoid and parasphenoid only
rarely, thus differing from the arrangement in the Holostei.

The isolated teeth are generally conical in form and are ankylosed to
the bone that bears them. Such teeth are, with a few exceptions such
as _Balistes_, not imbedded in sockets nor replaced vertically.

In some fish beak-like structures occur, formed partly of teeth,
partly of the underlying jaw bones. These beaks are of two kinds: (1)
In _Scarus_, the parrot fish, the premaxillae and dentaries bear
numerous small, separately developed teeth, which are closely packed
together and attached by their proximal ends to the bone, while their
distal ends form a mosaic. Not only the teeth but the jaws which bear
them are gradually worn away at the margins, while both grow
continuously along their attached edge. (2) In Gymnodonts, e.g.
_Diodon_, the beaks are formed by the coalescence of broad calcified
horizontal plates, which when young are free and separated from one
another by a considerable interval.

In some Teleosteans the differentiation of the teeth into biting teeth
and crushing teeth is as complete as in _Lepidosteus_. Thus in the
Wrasse (_Labrus_), the jaws bear conical slightly recurved teeth
arranged in one or two rows, with some of the anterior ones much
larger than the rest. The bones of the palate are toothless, while
both upper and lower pharyngeal bones are paved with knob-like
crushing teeth; such pharyngeal teeth occur also in the Carp but are
attached only to the lower pharyngeal bone, the jaw bones proper being
toothless.

In DIPNOI the arrangement of the teeth is very similar to that in
Holocephali. The mandible bears a single pair of grinding teeth
attached to the splenials, and a corresponding pair occur on the
palato-pterygoids. In front of these there are a pair of small conical
vomerine teeth loosely attached to the ethmoid cartilage. The
palato-pterygoid teeth of _Ceratodus_ are roughly semicircular in
shape with a smooth convex inner border, and an outer border bearing a
number of strongly marked ridges. The teeth of the extinct Dipteridae
resemble those of _Ceratodus_ but are more complicated.

ENDOSKELETON.

SPINAL COLUMN[41].

The spinal column of fishes is divisible into only two regions, a
caudal region in which the haemal arches or ribs meet one another
ventrally, and a precaudal region in which they do not meet.

The various modifications of the spinal column in fishes can be best
understood by comparing them with the arrangement in the simplest type
known, namely _Amphioxus_. In _Amphioxus_ the notochord is immediately
surrounded by a structureless cuticular layer, the _chordal sheath_.
Outside this is the _skeletogenous layer_, which in addition to
surrounding the notochord and chordal sheath embraces the nerve cord
dorsally, and laterally sends out septa forming the _myomeres_.

The CARTILAGINOUS GANOIDS[42] _Acipenser_, _Polyodon_ and
_Scaphirhynchus_ are the simplest fishes as regards their spinal
column. The notochord remains permanently unconstricted and is
enclosed in a chordal sheath, external to which is the skeletogenous
layer. In this layer the development of cartilaginous elements has
taken place. In connection with each _neuromere_, or segment as
determined by the points of exit of the spinal nerves, there are
developed two pairs of ventral cartilages, the ventral arches
(basiventralia) and intercalary pieces (interventralia); and at least
two pairs of dorsal pieces, the neural arches (basidorsalia) and
intercalary pieces (interdorsalia). The lateral parts of the
skeletogenous layer do not become converted into cartilage, so there
are no traces of vertebral centra. The ventral or haemal arches meet
one another ventrally and send out processes to protect the ventral
vessels. The neural arches do not meet, but are united by a
longitudinal elastic band.

In Cartilaginous ganoids the only indications of metameric
segmentation are found in the neural and haemal arches. The case is
somewhat similar with the Holocephali and Dipnoi.

In the HOLOCEPHALI the notochord grows persistently throughout life,
and is of uniform diameter throughout the whole body except in the
cervical region and in the gradually tapering tail. The chordal sheath
is very thick and includes a well-marked zone of calcification which
separates an outer zone of hyaline cartilage from an inner zone. There
are also a number of cartilaginous pieces derived from the
skeletogenous layer which are arranged in two series, a dorsal series
forming the neural arches and a ventral series forming the haemal
arches. These do not, except in the cervical region, meet one another
laterally round the notochord and form centra. To each neuromere there
occur a pair of basidorsals, a pair of interdorsals, and one or two
supradorsals. In the tail the arrangement is irregular.

In the DIPNOI as in the Holocephali the notochord grows persistently
and uniformly, and the chordal sheath is thick and cartilaginous
though there are no metamerically arranged centra. The neural and
haemal arches and spines are cartilaginous and interbasalia
(intercalary pieces) are present. The basidorsalia and basiventralia
do not in _Ceratodus_ meet round the notochord and enclose it except
in the anterior part of the cervical and posterior part of the caudal
region.

In ELASMOBRANCHII the chordal sheath is weak and the skeletogenous
layer strong. Biconcave cartilaginous vertebrae are developed, and as
is the case in most fishes, constrict the notochord _vertebrally_.

Two distinct types of vertebral column can be distinguished in
Elasmobranchs[43]:

1. In many extinct forms and in the living Notidanidae, _Cestracion_,
and _Squatina_, the dorsal and ventral arches do not meet one another
laterally round the centrum, and consequently readily come away from
it.

2. In most living Elasmobranchs the arches meet laterally round the
centrum.

The vertebrae are never ossified but endochondral calcification nearly
always takes place, though it very rarely reaches the outer surface of
the vertebrae. Elasmobranchs are sometimes subdivided into three
groups according to the method in which this calcification takes
place:

1. =Cyclospondyli= (_Scymnus_, _Acanthias_), in which the calcified
matter is deposited as one ring in each vertebra.

2. =Tectospondyli= (_Squatina_, _Raia_, _Trygon_), in which there are
several concentric rings of calcification.

3. =Asterospondyli= (Notidanidae, _Scyllium_, _Cestracion_), in which
the calcified material instead of forming one simple ring, extends out
in a more or less star-shaped manner.

In _Heptanchus_ the length of the vertebral centra in the middle of
the trunk is double that in the anterior and posterior portions, and
as the length of the arches does not vary, the long centra carry more
of them than do the short centra.

In many Rays the skull articulates with the vertebral column by
distinct occipital condyles.

In BONY GANOIDS the skeletogenous layer becomes calcified
ectochondrally in such a way that the notochord is pinched in at
intervals, and distinct vertebrae are produced. Ossification of the
calcified cartilage rapidly follows. In _Amia_ the vertebrae are
biconcave, in _Lepidosteus_ they are opisthocoelous, cup and ball
joints being developed between the vertebrae in a manner unique among
fishes. The notochord entirely disappears in the adult _Lepidosteus_,
but at one stage in larval life it is expanded vertebrally and
constricted intervertebrally in the manner usual in the higher
vertebrata, but unknown elsewhere among fishes.

The tail of _Amia_ is remarkable from the fact that as a rule to each
neuromere, as determined by the exit of the spinal nerves, there are
two centra, a posterior one which bears nothing, and an anterior one
which bears the neural and haemal arches, these being throughout the
vertebral column connected with the centra by cartilaginous discs.

In most TELEOSTEANS but not in the Plectognathi the neural arches are
continuous with the centra, which are nearly always deeply biconcave.

In some cases many of the anterior vertebrae are ankylosed together
and to the skull. The vertebrae often articulate with one another by
means of obliquely placed flattened surfaces, the zygapophyses. The
centrum in early stages of development is partially cartilaginous, but
the neural arches and spines in the trunk at any rate, pass directly
from the membranous to the osseous condition.

FINS.

The most primitive fins are undoubtedly the unpaired ones, which
probably originally arose as ridges or folds of skin along the
mid-dorsal line of the body, and passed thence round the posterior end
on to the ventral surface, partially corresponding in position and
function to the keel of a ship.

In long 'fish' which pass through the water with an undulating motion
such simple continuous fins may be the only ones found, as in
_Myxine_. To support these median fins skeletal structures came to be
developed; these show two very distinct forms, viz. cartilaginous
endoskeletal pieces, the _radiale_, and horny exoskeletal fibres, the
_fin-rays_. Mechanical reasons caused the fin to become concentrated
at certain points and reduced at intervening regions. Thus a terminal
caudal fin arose and became the chief organ of propulsion, and the
dorsal and ventral fins became specialised to act as balancing organs.

In some of the earlier Elasmobranchs, the Pleuracanthidae, the
endoskeletal cartilaginous radiale are directly continuous with
outgrowths from the dorsal and ventral arches of the vertebrae, and
form the main part of the fin. In later types of Elasmobranchs the
horny exoskeletal fin-rays have comparatively greater prominence. In
bony fish, as has been already stated, the horny fibres are replaced
by bony rays of dermal origin, and at the same time complete reduction
and disappearance of the cartilaginous radiale takes place.

THE CAUDAL FIN.

The caudal region of the spinal column in fishes is of special
importance. It is distinctly marked off from the rest of the spinal
column by the fact that the ventral or haemal arches meet one another
and are commonly prolonged into spines, while in the trunk region they
do not meet but commonly diverge from one another.

In some fish the terminal part of the caudal region of the spinal
column retains the same direction as the rest of the spinal column.
The blade of the caudal fin is then divided into two nearly equal
portions, and is said to be =diphycercal=. This condition is generally
regarded as the most primitive one; it occurs in the Ichthyotomi,
Holocephali, all living Dipnoi, _Polypterus_ and some extinct
Crossopterygii, and a few Selachii and Teleostei. It occurs also in
deep-sea fish belonging to almost every group, and under these
conditions obviously cannot be regarded as primitive, but must be
looked on as a feature induced by the peculiar conditions of life.

In the great majority of fish the terminal part of the caudal region
of the spinal column is bent dorsalwards, and the part of the blade of
the caudal fin which arises on the dorsal surface is much smaller than
is that arising on the ventral surface. Such a fin is said to be
=heterocercal=.

Strictly speaking all fish whose tails are not diphycercal have
heterocercal tails, but the term is commonly applied to two-bladed
tails in which the spinal column forms a definite axis running through
the dorsal blade, while the ventral blade is enlarged and generally
forms the functional part of the tail. Such heterocercal tails are
found in nearly all Elasmobranchii, together with the living
cartilaginous Ganoidei, and many extinct forms belonging to the same
order; _Lepidosteus_, _Amia_, and the Dipteridae among Dipnoi, have
tails which, though obviously heterocercal, are not two-bladed.

The vast majority of the Teleostei and some extinct Ganoidei have
heterocercal tails of the modified type to which the term =homocercal=
is applied. The hypural bones which support the lower half of the tail
fin become much enlarged, and frequently unite to form a wedge-shaped
bone which becomes ankylosed to the last ossified vertebral centrum.
The fin-rays then become arranged in such a way as to produce a
secondary appearance of symmetry. Some homocercal fish such as the
Perch have the end of the notochord protected by a calcified or
completely ossified sheath, the =urostyle=, to which several neural
and haemal arches may be attached, and which becomes united with the
centrum of the last vertebra; in others such as the Salmon the end of
the notochord is protected only by laterally placed bony plates.

THE SKULL.

It is often impossible to draw a hard and fast line between the
cranium and the vertebral column. This is the case for instance in
_Acipenser_ (fig. 18, 16) among Chondrostei, in _Amia_ among Holostei,
and in _Ceratodus_ and _Protopterus_ among Dipnoi. The occipital
region of the skull in _Amia_ is clearly formed of three cervical
vertebrae whose centra have become absorbed into the cranium, while
the neural arches and spines are still distinguishable.

The simplest type of cranium is that found in ELASMOBRANCHS: it
consists of a simple cartilaginous box, which is generally immovably
fixed to the vertebral column, though in some forms, like _Scymnus_
and _Galeus_, a joint is indicated, and in others, such as the
Rays, one is fairly well developed. The cranium in Elasmobranchs
is never bony, though the cartilage is sometimes calcified. It is
drawn out laterally into an antorbital process in front of the eye,
and a postorbital process behind it. The nasal capsules are always
cartilaginous, and the eye, as a general rule, has a cartilaginous
sclerotic investment. The cranium is often prolonged in front into
a rostrum which is enormously developed in _Pristis_ and some Rays.
The cartilaginous roof of the cranium is rendered incomplete by the
presence of a large hole, the anterior fontanelle.

[Illustration FIG. 16. A. SKULL OF _Notidanus_ × 1/2 (Brit. Mus.). B.
SKULL OF _Cestracion_ × 1/3 (after GEGENBAUR). In neither case are the
branchial arches shown.

    1. rostrum.
    2. olfactory capsule.
    3. ethmo-palatine process.
    4. palatine portion of palato-pterygo-quadrate bar.
    5. quadrate portion of bar.
    6. Meckel's cartilage.
    7. teeth.
    8. labial cartilage.
    9. hyomandibular.
    10. postorbital process.
    II. optic foramen.]

Two pairs of labial cartilages (fig. 16, B, 8) are often present. They
lie imbedded in the cheeks outside the anterior region of the jaws,
and are specially large in _Squatina_.

As regards the visceral arches[44] the simplest and most primitive
condition of the jaws is that of the Notidanidae, in which the
mandibular and hyoid arches are entirely separate. In these primitive
fishes the palato-pterygo-quadrate bar articulates with the
postorbital process (fig. 16, 10), while further forwards it is united
to the cranium by the ethmo-palatine ligament. The hyoid arch is small
and is broadly overlapped by the mandibular arch. The term
=autostylic= is used to describe this condition of the suspensorium.
From this condition we pass in the one direction to that of
_Cestracion_ (fig. 16, B), in which the whole of the palato-pterygo
quadrate bar has become bound to the cranium, and in the other to that
of _Scyllium_. In _Scyllium_ (fig. 6), while the ethmo-palatine
ligament is retained, the postorbital articulation of the palato
pterygo-quadrate has been given up, so that the palato-pterygo
quadrate comes to abut on the hyomandibular and is attached to it by
ligaments. The pre-spiracular ligament (fig. 16, 20) running from the
auditory capsule also assists in supporting the jaws.

Lastly we come to the purely =hyostylic= condition met with in Rays,
in which the mandibular arch is entirely supported by the
hyomandibular. In some Rays the hyoid is attached to the posterior
face of the hyomandibular near its proximal end, and may even come to
articulate with the cranium.

The =visceral arches of Elasmobranchs= may be summarised as follows:--

1. The =mandibular arch=, consisting of a much reduced dorsal portion,
the pre-spiracular ligament, and a greatly developed ventral portion
from which both upper and lower jaws are derived. The mandible
(Meckel's cartilage) is the original lower member of the mandibular
arch, and from it arises an outgrowth which forms the upper jaw or
palato-pterygo-quadrate bar. In _Scymnus_ this bears a few
branchiostegal rays.

2. The =hyoid arch=, which consists of the hyomandibular and the
hyoid, and bears branchiostegal rays on its posterior face.

[Illustration FIG. 17. DORSAL VIEW OF THE BRANCHIAL ARCHES OF
_Heptanchus_. (From GEGENBAUR).

    1. basi-hyal.
    2. cerato-hyal.
    3. second hypo-branchial.
    4. first cerato-branchial.
    5. first epi-branchial.
    6. first pharyngo-branchial.
    7. pharyngo-branchial, common to the sixth and seventh arches.
    8. basibranchial of second arch.
    9. basibranchial, common to the sixth and seventh arches.]

3. The =branchial arches=, generally five in number, all of which
except the last bear branchiostegal rays. In the Notidanidae the
number of branchial arches is increased beyond the normal series,
thus in _Hexanchus_ there are six, and in _Heptanchus_ seven. There
are six also in _Chlamydoselache_ and _Protopterus_.

4. The so-called external branchial arches which are cartilaginous
rods attached to all the visceral arches. They are especially large in
_Cestracion_.

The skull in HOLOCEPHALI is entirely cartilaginous. The
palato-pterygo-quadrate bar is fixed to the cranium, and to it the
mandible articulates. There is a well-marked joint between the skull
and the spinal column.

In living Cartilaginous GANOIDS the primitive cartilaginous cranium is
very massive, and is greatly prolonged anteriorly, while posteriorly
it merges into the spinal column. Although it is mainly cartilaginous
a number of ossifications take place in the skull, and membrane bones
are now found definitely developed, especially in connection with the
roof of the cranium. In _Acipenser_ (fig. 18) the ossifications in the
cartilage include the pro-otic, which is pierced by the foramen for
the fifth nerve, the alisphenoid, orbitosphenoid, ectethmoid,
palatine, pterygoid, meso-pterygoid, hyomandibular (fig. 18, 11),
cerato-hyal, all the cerato-branchials, and the first two
epi-branchials. Most of these structures are, however, partly
cartilaginous, though they include an ossified area. The membrane
bones too of _Acipenser_ are very well developed, they include a bone
occupying the position of the supra-occipital, and form a complete
dorsal cephalic shield. Resting on the ventral surface are a vomer and
a very large parasphenoid (fig. 18, 3). There is a bony operculum
attached to the hyomandibular, and membrane bones representing
respectively the maxilla and dentary are attached to the jaws. The
suspensorium is most markedly hyostylic. The palato-pterygo-quadrate
bar has a very curious shape and is quite separate from the cranium.
It is connected to the hyomandibular by a thick symplectic ligament
containing a small bone homologous with the symplectic of
Teleosteans.

_Polyodon_ differs much from _Acipenser_, the membrane bones not being
so well developed though they cover the great cartilaginous snout.

[Illustration FIG. 18. LATERAL VIEW OF THE SKULL OF A STURGEON
(_Acipenser sturio_). Nearly all the membrane bones have been removed
(Brit. Mus.).

    1. nasal cavity.
    2. orbit.
    3. parasphenoid.
    4. vomer.
    5. pterygoid.
    6. maxilla. (The dotted line running from 6 passes into the mouth
        cavity.)
    7. dentary.
    8. symplectic.
    10. palatine.
    11. hyomandibular.
    12. pharyngo-branchial.
    13. epi-branchial.
    14. cerato-branchial.
    15. hypo-branchial.
    16. coalesced anterior vertebrae.
    17. inter-hyal.
    18. cerato-hyal.
    19. rib.]

The skull in _Polypterus_ (Crossopterygii) shows a great advance
towards the condition met with in Teleostei. The cranium remains to a
great extent unossified, and large dorsal and ventral fontanelles
pierce its walls. It is covered by a great development of membrane
bones, paired nasals, frontals, parietals, supra- and post-temporals,
and dermo-supra-occipitals among others being present. The
palato-pterygo-quadrate bar is fused to the cranium, and in connection
with it the following paired membrane bones appear, palatine, ecto-,
meso- and meta-pterygoid, and further forwards jugal, vomer, maxilla
and premaxillae. The membrane bones developed in connection with each
ramus of the mandible are the dentary, angular, and splenial, in
addition to the cartilage bone the articular. Several large opercular
bones occur. There are also a pair of large jugular or gular plates,
and several large opercular bones.

In Bony Ganoids both cartilage bone and membrane bone is well
developed. The pro-otics and exoccipitals are well ossified, but the
supra-occipital and pterotics are not. Lateral ethmoids are developed,
and there are ossifications in the sphenoidal region which vary in
different forms. The place of the cartilaginous palato-pterygo
quadrate is taken by a series of bones, the quadrate behind and the
palatine, ecto-, meso- and meta-pterygoids in front. In _Lepidosteus_,
however, the palatine and pterygoid are membrane bones, as they are in
_Polypterus_ and the Frog. Paired maxillae, premaxillae, vomers and a
parasphenoid occur forming the upper jaw and roof of the mouth, and a
series of membrane bones are found investing the mandible and forming
the operculum.

In _Amia_[45] membrane bones are as freely developed as they are in
Teleosteans; they include on each side a squamosal, four opercular
bones, a lachrymal, a pre-orbital, one or two suborbitals, two large
postorbitals and a supratemporal; while investing the mandible,
besides the dentary, splenial, angular, and supra-angular, there is an
unpaired jugular. The articular too is double and a mento-meckelian
occurs. In _Amia_ teeth are borne on the premaxillae, maxillae, vomers,
palatines and pterygoids.

Bony Ganoids are the lowest animals in which squamosal bones are
found, and they do not occur in Teleosteans.

The suspensorium in bony Ganoids, as in the Chondrostei, is hyostylic,
and there are two ossifications in the hyomandibular cartilage, viz.
the hyomandibular, and the symplectic.

The skull of TELEOSTEI is very similar to those of _Lepidosteus_ and
_Amia_. Although the bony skull is greatly developed and very
complicated, much of the original cartilaginous cranium often
persists. Membrane bones are specially developed on the roof of the
skull where they include the parietal, frontal, and nasal bones. The
same bones are developed in connection with the upper jaw and roof of
the mouth as in bony Ganoids, but only two membrane bones occur in the
lower jaw, viz. the angular and dentary. A number of large
ossifications take place in the cartilage of the auditory capsules. In
some forms parts of the last pair of branchial arches are broadened
out and form the pharyngeal bones which bear teeth. The opercular
bones and those of the upper and lower jaws are quite comparable to
those of bony Ganoids.

A full account of the Teleostean skull has been given in the case of
the Salmon (pp. 87-96) and the Cod (pp. 96-101).

In DIPNOI the skull is mainly cartilaginous, but both cartilage- and
membrane-bone occur also. Cartilage-bone is found in the ossified
exoccipitals, while of membrane-bones _Protopterus_ has among unpaired
bones a fronto-parietal, a median ethmoid, and a parasphenoid, and
among paired bones nasals and large supra-orbitals. The skull of
_Ceratodus_ (fig. 19) has an almost complete roof of membrane bones,
including some whose homology is doubtful. The ethmo-vomerine region
is always cartilaginous, but bears small teeth. The palato-pterygo
quadrate bar is ossified and firmly united to the cranium, and the
mandible articulates directly with it (autostylic). Membrane bones are
freely developed in connection with the mandible, dentary, splenial,
and angular bones being all present. There are two opercular bones.

In the extinct Dipteridae the cranium is very completely covered with
plates of dermal bone, and the skeleton in general is more ossified
than is the case in recent Dipnoi.

Six pairs of branchial arches occur in _Protopterus_; _Ceratodus_ and
_Lepidosiren_ have five, like most other fish. The branchial arches
bear gill rakers.

[Illustration FIG. 19. DORSAL (TO THE LEFT) AND VENTRAL (TO THE RIGHT)
VIEWS OF THE CRANIUM OF _Ceratodus miolepis_ (after GÜNTHER).

    1. cartilaginous part of the quadrate with which the mandible
        articulates.
    2. scleroparietal.
    3. frontal.
    4. ethmoid.
    5. nares.
    6. orbit.
    7. pre-opercular (squamosal).
    8. second rib.
    9. first rib.
    10. vomerine tooth.
    11. palato-pterygoid tooth.
    12. palato-pterygoid.
    13. parasphenoid.
    14. interopercular.]

RIBS.

As has been already mentioned (p. 24), although ribs commonly appear
to be the cut-off ends of the transverse processes, they are really
elements derived from the ventral or haemal arch.

In Elasmobranchii and other cartilaginous fish they have the form of
small cartilaginous structures imperfectly separated from the
diverging halves of the ventral arch, and are often absent.

In Teleostei and bony Ganoids they often have different attachments in
different parts of the body. In the tail region they are not
differentiated from the two halves of the ventral arch, which meet in
the middle line, and are prolonged into a haemal spine. In the
posterior trunk region they sometimes form distinct processes
diverging from the two halves of the ventral arch; while further
forward they may shift their attachment so as to arise from the dorsal
side of the two halves of the ventral arch and at some distance from
their ends, which now diverge as ventri-lateral processes.

APPENDICULAR SKELETON.

PECTORAL GIRDLE.

The simplest type of pectoral girdle is found in Elasmobranchs. It is
entirely cartilaginous and consists of a curved ventrally-placed rod,
ending dorsally in two horn-like scapular processes which are
sometimes attached to the cranium or vertebral column. In Rays the
shoulder girdle is very large, and has a distinct suprascapular
portion forming a broad plate attached to the neural spines of the
vertebrae. There is often a cup-like glenoid cavity for the
articulation of the limb; this cavity is specially large in Rays and
is much pierced by holes. In Dipnoi the cartilaginous girdle still
occurs, but on it there is a deposit of membrane bone forming the
clavicle, infraclavicle, and supra-clavicle. These bones, which with
the exception of the clavicle, are unknown in higher vertebrates, are
better developed in Ganoids, and best of all in Teleosteans. They are
connected by the supratemporal with the epi-otic and opisthotic
regions of the cranium. Owing to this development of dermal bone, the
original cartilaginous arch becomes much reduced, but ossifications
representing the scapula and coracoid occur in bony Ganoids and
Teleosteans.

PELVIC GIRDLE.

In Elasmobranchs the pelvic girdle consists of a short ventral rod of
cartilage representing the ischium and pubis, which does not send up
dorsal iliac processes. In _Chimaera_ the pelvic girdle has a
flattened pointed iliac portion, and ventrally an unpaired movable
cartilaginous plate which bears hooks and is supposed to be copulatory
in function. Claspers of the usual type are present as well. The
Dipnoi have a primitive kind of pelvis in the form of a cartilaginous
plate lying in the mid-ventral line and drawn out into three horns
anteriorly. In Ganoids the pelvis has almost entirely disappeared,
though small cartilaginous vestiges of it remain in _Polypterus_. In
Teleosteans even these vestiges are gone, and in these fish and
Ganoids the place of the pelvis is taken by the enlarged basi-pterygia
(meta-pterygia) of the fins.

PAIRED FINS[46].

As regards the origin of the limbs or paired fins of fishes there are
two principal views. One view, that of Gegenbaur, considers that limbs
and their girdles are derived from visceral arches which have migrated
backwards. The other view, which probably now has the greater number
of supporters, considers that the paired fins of fishes are of
essentially the same nature as the median fins.

According to Gegenbaur's view[47] the =archipterygium= of _Ceratodus_
(fig. 20) represents the lowest type of fin; it consists of a central
cartilaginous axis bearing a large number of radiale. The dorsal or
pre-axial radiale are more numerous than the ventral or postaxial, and
at the margin of the fin[48] the cartilaginous endoskeletal radiale
are replaced by horny exoskeletal fin-rays.

[Illustration FIG. 20. LATERAL VIEW OF THE SKELETON OF _Ceratodus
miolepis_ (after GÜNTHER).

    1. ethmoid.
    2. scleroparietal.
    3. frontal.
    4. cartilaginous posterior part of cranium.
    5. pre-opercular (squamosal).
    6. opercular.
    7. suborbital.
    8. orbit.
    9. pectoral girdle.
    10. proximal cartilage of pectoral fin.
    11. pectoral fin.
    12. pelvic girdle.
    13. pelvic fin.
    14. spinal column.
    15. caudal fin (diphycercal).]

It is impossible here to give a full discussion of the rival views,
but some of the points which support Gegenbaur's view may be
mentioned. The fact that migration of visceral arches has to be
assumed is no difficulty, as it is obvious that migration in the
opposite direction has taken place in many Teleosteans such as the
Cod, whose pelvic fins are attached to the throat in front of the
pectorals. If migration did take place, the pelvic fins being older
than the pectoral should be the more modified, and this is the case.
Again, if the pectoral girdle is a modified branchial arch, it must at
some period have carried a gill, and in _Protopterus_ it does bear a
vestigial gill.

According to the view more prevalent at the present time, the paired
fins have been derived from two continuous folds of skin and their
skeletal supports running forward from the anal region along the sides
of the body, their character being similar to the fold that gave rise
to the median fins. In support of this view it may be argued that the
paired and unpaired fins are often identical in structure, and that
some Elasmobranch embryos do show a ridge running between the pectoral
and pelvic fins. Then from this continuous fold two pairs of smaller
folds may have been specialised off, and in each a number of
cartilaginous radiale may have been developed. The fin of
_Cladoselache_ from the Carboniferous of Ohio apparently illustrates
this condition. It consists of certain basal pieces which do not
project beyond the body wall and bear a number of unsegmented
cartilaginous radiale, which show crowding together and are sometimes
bifurcated distally; they extend throughout the whole fin from the
body wall to the margin. From this fin the archipterygium might be
easily derived by the enlargement of one of the middle radiale and the
segmentation and partial fusion of them all.

Whether the archipterygium be a primitive or secondary type of fin,
when it is once reached it is easy to derive all the other types from
it. The fins of the other living Dipnoi,--_Protopterus_ and
_Lepidosiren_--are simply archipterygia from which the radiale have
almost or completely disappeared, leaving only the segmented axes.
Archipterygia too are found in the pectoral fins of the Ichthyotomi,
but the postaxial radiale are much reduced.

The =ichthyopterygium=, or type of fin, characteristic of many modern
Elasmobranchs such as _Scyllium_, may have been derived from the
archipterygium by the gradual reduction of the rays on the postaxial
side of the axis and their condensation on the pre-axial side. The
Ichthyotomi such as _Xenacanthus_ show one stage in the reduction of
the postaxial rays, and a further stage is seen in the Notidanidae and
some other sharks like _Scymnus_ and _Acanthias_, in which a few
postaxial rays still remain. The condensation of the pre-axial rays
when further continued leads to one of the rays getting an attachment
to the girdle. Thus the fin comes to articulate with the girdle by two
basalia or basal pieces; a third attachment is formed in the same way
and the three basalia are called respectively pro-, meso-, and
meta-pterygia. By some authors the meta-pterygium and by others the
meso-pterygium is regarded as homologous with the axis of the
archipterygium.

The pectoral fins of Elasmobranchs vary very much in their mode of
attachment. In some of the sharks, including the Notidanidae and
_Scyllium_, all three basalia articulate with the pectoral girdle,
while in others such as _Cestracion_ the meta-pterygium is excluded.
In Rays the propterygium and the meta-pterygium are long and narrow
and diverge much from one another; other basalia work their way in
between the meso-pterygium and meta-pterygium, and come to articulate
with the pectoral girdle. Sometimes they fuse and form a second
meso-pterygium. The radiale are greatly elongated and are segmented.

In _Chimaera_ all three basalia are present, but the meso-pterygium
is shifted and does not articulate with the pectoral girdle[49].

In _Acipenser_ and _Polyodon_ the pectoral fin is built on the same
type as in Elasmobranchs, but becomes modified from the fact that the
propterygium is replaced by dermal bone which forms a large =marginal
ray=. Extra meso-pterygia are formed in the same way as in Rays.

In _Polypterus_ the pro-and meta-pterygia have ossified while the
meso-pterygium remains chiefly cartilaginous; the fin-rays are also
chiefly ossified.

In _Amia_, _Lepidosteus_, and certain Teleosteans like _Salmo_, not
only the propterygium but the meso-pterygium is almost suppressed by
the marginal ray.

In the great majority of Teleosteans a still further stage is reached,
the endoskeletal elements, the basalia and radiale are almost entirely
suppressed and the fin comes to consist entirely of ossified fin-rays
of dermal origin.

In some Teleosteans--_Exocaetus_, a herring, and _Dactylopterus_, a
gurnard--the pectoral fins are so enormously developed that by means
of them the fish is able to fly through the air for considerable
distances. The skeleton of these great fins is almost entirely
composed of dermal bone.

PELVIC FIN.

The pelvic fin is almost always further removed from the
archipterygial condition, and is in general more modified than is the
pectoral. Thus in the Ichthyotomi, while the pectoral fins are
archipterygia similar to those of _Ceratodus_, the pelvic fins consist
of an axis bearing rays on the postaxial side only, and prolonged
distally into a clasper. In Dipnoi however the pelvic fins are very
similar to the pectoral. In Elasmobranchs the meso-pterygium is
missing, the propterygium is small or absent, and the fin is mainly
composed of the meta-pterygium (generally called basi-pterygium) and
its radiale. The males in Elasmobranchii and Holocephali have the
distal end of the meta-pterygium prolonged into a clasper.

In Ganoids and in Teleosteans the loss of the pelvic girdle causes the
pelvic fin to be still further removed from the primitive state. There
is always a large basi-pterygium which lies imbedded in the muscles
and meets its fellow at its proximal end. In Cartilaginous Ganoids it
has a secondary segmentation. Its relation to its fellow is subject to
much variation in Teleosteans, sometimes as in the Perch the two are
in contact throughout, sometimes as in the Salmon they meet distally
as well as proximally, but are elsewhere separated by a space,
sometimes as in the Pike and Bony Ganoids they diverge widely. The
radiale are articulated to the basi-pterygium. In Cartilaginous
Ganoids and _Polypterus_ they are well developed, in other Ganoids and
in Teleosteans they are in the main replaced by dermal fin-rays.

In some Teleosteans such as the Cod the pelvic fins have migrated from
their usual position and come to be attached to the throat in front of
the pectoral fins. Fish with this arrangement are grouped together as
=jugulares=.

FOOTNOTES:

[39] The following general works on fishes may be referred to:
Bashford Dean, _Fishes, Living and Fossil_, New York, 1895. A.
Günther, _An Introduction to the Study of Fishes_, Edinburgh, 1880.
A.A.W. Hubrecht and M. Sagemehl, _Fische_ in Bronn's _Classen und
Ordnungen des Thierreichs_, Band VI. Leipzig, 1876.

[40] See W.G. Ridewood, _Nat. Sci._ vol. VIII. 1896, p. 380. Full
references are there given to the literature of the subject.

[41] See H. Gadow and E.C. Abbott, _Phil. Trans._ vol. 186 (1895) B.
pp. 163-221.

[42] C. Hasse, _Zeitschr. wiss. Zool._ LVII. 1893, p. 76.

[43] C. Hasse, _Das natürliche System der Elasmobranchier auf
Grundlage des Baues und der Entwickelung ihrer Wirbelsäule_, Jena,
1879 and 1885, and "Die fossilen Wirbel, Morph. Studien I.-IV.,"
_Morphol. Jahrb. Bd._ II., III. and IV. 1876-78.

[44] See H.B. Pollard, _Anat. Anz._ X. 1894.

[45] T.W. Bridge, "The Cranial Osteology of _Amia calva_," _J. Anat.
Physiol. norm. path._ 1876, vol. XI. p. 605. R. Shufeldt, "The
Osteology of _Amia calva_," _Ann. Rep. of the Commissioner for Fish
and Fisheries_, Washington, 1885.

[46] A. Smith Woodward, _Nat. Sci._ vol. I. 1892, p. 28. Further
references are here given on the literature of the subject.

[47] C. Gegenbaur, Ueber das Archipterygium, _Jena Zeitschr. der
Wirbelthiere_, 2^e Heft, 1873, vol. 7, and _Morphol. Jahrb._ XXII.
1894, p. 119.

[48] The fins of _Ceratodus_ are very variable, no two being exactly
alike. Sometimes even the main axis bifurcates. See W.A. Haswell,
_Linn. Soc. N. S. Wales_, vol. VII. 1882.

[49] Some of these views with regard to the homologies of the parts of
the fins are not accepted by all anatomists.

CHAPTER IX.

CLASS II. AMPHIBIA[50].

AMPHIBIA differ markedly from Pisces in the fact that in the more
abundant and familiar forms the skin is naked, and that when the
integument is prolonged into median fins they are devoid of fin-rays.
The notochord may persist, but bony vertebral centra are always
developed. These are sometimes biconcave, sometimes procoelous,
sometimes opisthocoelous. There is only one sacral vertebra, except in
rare cases. The cartilaginous cranium persists to a considerable
extent but is more or less replaced by cartilage bone, and overlain by
membrane bone. The basi-occipital is not completely ossified, and the
skull articulates with the vertebral column by means of two occipital
condyles formed by the exoccipitals.

There is a large parasphenoid, but there are no ossifications in the
basisphenoidal, presphenoidal, and alisphenoidal regions. In most
cases the epi-otics and opisthotics are ossified continuously with the
exoccipitals.

The palato-pterygo-quadrate bar is firmly united to the cranium, so
the skull is autostylic. The palatines and pterygoids are membrane
bones. Teeth are nearly always borne on the vomers and commonly on the
maxillae and premaxillae. There are no sternal ribs, and the sternum is
very intimately related to the pectoral girdle. There are no obturator
foramina. The limbs are as in the higher vertebrata, divisible into
upper arm, fore-arm, and manus (wrist and hand), and into thigh, shin,
and pes (ankle and foot) respectively. The posterior limb is, as a
rule, pentedactylate, but in nearly every case the pollex is vestigial
or absent.

_Order 1._ URODELA[51].

The Urodela are elongated animals with a naked skin, a persistent
tail, and generally four short limbs.

The vertebral centra are opisthocoelous or biconcave, and there are
numerous precaudal vertebrae. Portions of the notochord commonly
persist in the intervertebral spaces. In the skull there is no
sphenethmoid forming a ring encircling the anterior end of the brain,
its place being in many cases partly taken by a pair of
orbitosphenoids. There is no quadratojugal, and the quadrate is more
or less ossified. The mandible has a distinct splenial, and the
articular is ossified.

There is no definite tympanic cavity. The hyoid apparatus is
throughout life connected to the quadrate by ligament, and a large
basilingual plate does not occur. The ribs are short structures with
bifurcated proximal ends. In the pelvis the pubis remains
cartilaginous, and there is a bifid cartilaginous epipubis. The bones
of the fore-arm and shin remain distinct, and the manus never has more
than four digits.

_Suborder_ (1). ICHTHYOIDEA.

The vertebrae are amphicoelous, but the notochord remains but little
constricted throughout the whole length of the vertebral column. Three
or four branchial arches nearly always persist in the adult. The
cartilages of the carpus and tarsus remain unossified.

The Ichthyoidea may be subdivided again into two groups:--

A. _Perennibranchiata_, whose chief distinguishing skeletal characters
are that the skull is elongated, the premaxillae are not ankylosed, the
maxillae are vestigial or absent; there are sometimes no nasals, and
the palatines bear teeth;

    e.g. _Siren_, _Proteus_, _Menobranchus_.

B. _Derotremata_, whose chief distinguishing skeletal characters are
that there are large maxillae and nasals; teeth are borne by both
maxillae and premaxillae; there are no palatines; and both pectoral and
pelvic limbs are always present;

    e.g. _Amphiuma_, _Megalobatrachus_, _Cryptobranchus_.

_Suborder_ (2). SALAMANDRINA.

The vertebrae are opisthocoelous. The skull is broad, and teeth are
borne by both premaxillae and dentaries. Nasal bones are present. The
remains of only two branchial arches are found in the adult. The
carpus and tarsus are more or less ossified.

This suborder includes the Newts (_Molge_), Salamanders
(_Salamandra_), and _Amblystoma_.

_Order_ 2. LABYRINTHODONTIA[52].

These are extinct Amphibia with a greatly developed dermal
exoskeleton, which is generally limited to the ventral surface. The
body and tail are long and in some cases limbs are absent. The teeth
are pointed and often have the dentine remarkably folded. The
vertebrae are amphicoelous, and are generally well ossified. The skull
is very solid, and has a greatly-developed secondary roof which hides
the true cranium and is very little broken up by fossae. Paired
dermal supra-occipitals are found, and there is an interparietal
foramen. The epi-otics and opisthotics form a pair of bones distinct
from the exoccipitals. Four simple limbs of moderate length are
generally present, and in some cases all four limbs are
pentedactylate. Among the better known genera of Labyrinthodonts are
_Mastodonsaurus_, _Nyrania_, and _Archegosaurus_.

_Order 3._ GYMNOPHIONA[53].

These animals form a group of abnormal worm-like Amphibia having an
exoskeleton in the form of subcutaneous scales arranged in rings. The
vertebrae are biconcave and are very numerous; very few however belong
to the tail. The skull has a complete secondary bony roof, the
mandible bears teeth and has an enormous backward projection of the
angular. The hyoid arch has very slender cornua and no distinct body,
it is attached neither to the cranium nor to the suspensorium. The
ribs are very long and there are no limbs or limb girdles.

_Order 4._ ANURA.

These are tailless Amphibia, which except in a few instances, are
devoid of an exoskeleton. The vertebrae are as a rule procoelous, and
are very few in number. The post-sacral part of the spinal column
ossifies continuously, forming an unsegmented cylindrical rod, the
urostyle. Remains of the notochord persist, lying _vertebrally_, i.e.
enclosed within the centra of the several vertebrae, and not as in
Urodela lying between one vertebra and the next. The skull is very
short and wide. The mandible is almost always, if not invariably,
toothless.

The frontals and parietals on each side are united so as to form a
pair of fronto-parietals, and a girdle-like sphenethmoid is present.

The quadrate is not generally ossified. A predentary or
mento-meckelian bone is commonly present in the mandible, and a single
bone represents the angular and splenial. The branchial arches are
much reduced in the adult, and the distal ends of the cornua unite to
form a flat basilingual plate of a comparatively large size.

Ribs are very little developed. Clavicles are present. The ilia are
very greatly elongated. The anterior limb has four well-developed
digits and a vestigial pollex, and is of moderate length; the radius
and ulna have fused. The posterior limb is greatly elongated and is
pentedactylate; the tibia and fibula are fused, while the calcaneum
and astragalus are greatly elongated, and it is largely owing to them
that the length of the limb is so great. The group includes the Frogs
and Toads, the predominant Amphibia of the present time.

FOOTNOTES:

[50] T.H. Huxley, _Amphibia_ (_Encyclopaedia Britannica_).

[51] See R. Wiedersheim, _Morphol. Jahrb._ Bd. III. 1877, p. 459.

[52] See A. Fritsch, _Fauna der Gaskohle_, Prague, 1883-85-86, also
writings of Cope, Credner, Huxley, H. v. Meyer, Miall.

[53] See R. Wiedersheim, _Anatomie der Gymnophionen_, Jena, 1879.

CHAPTER X.

THE SKELETON OF THE NEWT (_Molge cristata_).

I. EXOSKELETON.

The skin of the Newt is quite devoid of any exoskeletal structures.
The only exoskeletal structures that the animal possesses are the
teeth, and these are most conveniently described with the
endoskeleton.

II. ENDOSKELETON.

The endoskeleton of the Newt, though ossified to a considerable
extent, is more cartilaginous than is that of the frog. It is
divisible into an =axial portion= including the vertebral column,
skull, ribs, and sternum, and an =appendicular portion= including the
skeleton of the limbs and their girdles.

1. THE AXIAL SKELETON.

A. THE VERTEBRAL COLUMN.

This consists of about fifty vertebrae arranged in a regular
continuous series. The first vertebra differs a good deal from any of
the others; the seventeenth or sacral vertebra and the eighteenth or
first caudal also present peculiarities of their own. The remaining
vertebrae are divided by the sacrum into an anterior series of =trunk=
vertebrae which bear fairly large ribs, and a posterior series of
=caudal= vertebrae, all of which except the first few are ribless.

THE TRUNK VERTEBRAE.

Any vertebra from the second to the sixteenth may be taken as a type
of the trunk vertebrae.

The general form is elongated and somewhat hour-glass shaped, and the
=centra= are convex in front and concave behind; an opisthocoelous
condition such as this is quite exceptional in Anura. The =notochord=
may persist intervertebrally[54], but in the centre of each vertebra
it becomes greatly constricted or altogether obliterated, and replaced
by marrow. The superficial portion of the centrum is ossified, while
the articular surfaces are cartilaginous. The =neural arches= are low
and articulate together by means of =zygapophyses= borne on short
diverging processes. The anterior zygapophyses look upwards, the
posterior downwards. Each neural arch is drawn out dorsally into a
very slight cartilaginous =neural spine=.

On each centrum, at a little behind the middle line, there arise a
pair of short backwardly-directed =transverse processes=; each of
which becomes divided into two slightly divergent portions, a dorsal
portion which meets the tubercular process of the rib and is derived
from the neural arch, and a ventral portion which meets the capitular
process of the rib and is derived from the ventral or haemal arch. The
division between these two parts of the transverse processes can be
traced back as far as the sacrum.

The =first vertebra= as already mentioned differs much from all the
others. It has no ribs, and presents anteriorly two slightly divergent
concave surfaces which articulate with the occipital condyles of the
skull. Between these surfaces the dorsal portion of the anterior face
of the centrum is drawn out into a prominent =odontoid process=, the
occurrence of which renders it probable that the first vertebra of
the newt is really the axis, and that the atlas with the exception of
the odontoid process has become fused with the skull. The sacral
vertebra or =sacrum= differs from the vertebrae immediately in front
of it only in the fact that its transverse processes are stouter and
more obviously divided into dorsal and ventral portions.

THE CAUDAL VERTEBRAE.

The =caudal vertebrae= are about twenty-four in number. The anterior
ones have hour-glass shaped centra, and short backwardly-directed
transverse processes. The middle and posterior ones have rather
shorter centra, and are without transverse processes. The neural
arches resemble those of the trunk vertebrae, but each is drawn out
into a rather high cartilaginous neural spine abruptly truncated
anteriorly. All the caudal vertebrae except the first have also a
haemal arch, which is very similar to the neural arch, and is drawn
out into a haemal spine quite similar to the neural spine. Both neural
and haemal arches are ossified continuously with the centra.

B. THE SKULL.

The skull of the newt is divisible into three principal parts:--

(1) an axial part, the =cranium proper=, which encloses the brain and
to which

(2) the =capsules= of the =auditory and olfactory sense organs= are
fused;

(3) the skeleton of the =jaws and hyoid apparatus=. The skull is much
flattened and expanded, though not so much as in the frog.

(1) THE CRANIUM PROPER.

The =cranium proper= or =brain case= is an unsegmented tube which
remains partly cartilaginous, and is partly converted into cartilage
bone, partly sheathed by membrane bone. The roof and floor of the
cartilaginous cranium are, as is the case also in the frog, pierced
by holes or fontanelles, and these are so large that the main part of
the roof and floor comes to be formed by membrane bone.

Two pairs of large ossifications take place in the cranial walls. Of
these the more posterior on each side represents the =exoccipital= and
all three =periotic= bones. It bears a small convex patch of cartilage
for articulation with the atlas, and with its fellow forms the
boundary of the foramen magnum.

Two foramina pierce the exoccipital just in front of the occipital
condyle and transmit respectively the glossopharyngeal and
pneumogastric (fig. 21, X) nerves. Lying laterally to these nerve
openings is seen a patch of cartilage, the =stapes=, which is
homologous with the stapes or proximal element of the columellar chain
in the frog. Further forward in front of the stapes is the small
opening for the exit of the facial nerve, and seen in a lateral view
close to the orbitosphenoid, that for the trigeminal (fig. 21, C, 5).

In front of these large bones the lateral parts of the cranial walls
remain cartilaginous for a short distance, and then there follow two
elongated bones, the =orbitosphenoids= (fig. 21, B and C, 11), pierced
by the foramina for the exit of the optic nerves. These bones partly
correspond to the sphenethmoid of the frog.

The _membrane bones_ connected with the cranium are the _parietals_,
_frontals_ and _prefronto-lachrymals_ on the dorsal surface, and the
_parasphenoid_ on the ventral surface.

The _parietals_ (fig. 21, A and C, 6) roof over the posterior part of
the great dorsal fontanelle and overlap the exoccipito-periotics. They
meet one another along a sinuous suture in the middle line, as do also
the _frontals_ which overlap them in front. The _frontals_ and
_parietals_ both extend for a short distance down the sides of the
cranium and meet the orbitosphenoids. The _prefronto-lachrymals_ (fig.
21, A and C, 7) connect the frontals with the maxillae.

[Illustration FIG. 21. A DORSAL, B VENTRAL, AND C LATERAL VIEWS OF THE
SKULL OF A NEWT (_Molge cristata_) × 2-1/2 (after PARKER).

The cartilage is dotted, the cartilage bones are marked with dots and
dashes, the membrane bones are left white.

    1. premaxillae.
    2. anterior nares.
    3. posterior nares.
    4. nasal.
    5. frontal.
    6. parietal.
    7. prefronto-lachrymal.
    8. maxillae.
    9. vomero-palatine.
    10. parasphenoid.
    11. orbitosphenoid.
    12. pterygoid.
    13. squamosal.
    14. pro-otic region of exoccipito-periotic.
    15. quadrate.
    16. quadrate cartilage.
    17. exoccipital region of exoccipito-periotic.
    18. articular.
    19. articular cartilage.
    20. dentary.
    21. splenial.
    22. middle narial passage.
    II. V. VII. IX. X. foramina for the exit of cranial nerves.]

On the ventral surface is the large _parasphenoid_ (fig. 21, B, 10),
which is widest behind and overlapped anteriorly by the
vomero-palatines.

(2) THE SENSE CAPSULES.

The =auditory capsules= become almost completely ossified continuously
with the exoccipitals; they have been already described.

The =nasal capsules= are large and quite unossified though they are
overlain by membrane bone. They appear on the dorsal surface between
the anterior nares and the nasal process of the premaxillae. They
enclose the nasal organs, bound the inner side of the anterior narial
opening, and are connected with one another posteriorly by a
cartilaginous area.

Developed in connection with the nasal capsules are a pair of rather
large _nasals_ (fig. 21, A and C, 4), which lie on the dorsal surface
immediately in front of the frontals. Each forms part of the posterior
boundary of one of the anterior nares, and the two are separated from
one another in the middle line by the nasal process of the premaxillae
(fig. 21, A, 1), and the opening of the =middle narial passage= (fig.
21, A and B, 22), which passes right through the skull.

On the ventral surface of the skull and forming the greater part of
the boundary of the posterior nares are two large bones, the
_vomero-palatines_ (fig. 21, B and C, 9). Each consists of a wide
anterior portion, partly separated from its fellow in the middle line
by the ventral opening of the middle narial passage, and of a long
pointed posterior portion which is separated from its fellow by the
_parasphenoid_, and bears a row of small pointed teeth formed of
dentine capped with enamel.

(3) THE JAWS.

The =upper jaw= of the newt is a discontinuous structure divided into
two parts, an anterior part which consists of membrane bones, the
_maxillae_ and _premaxillae_, and a posterior part which remains mainly
cartilaginous.

The _premaxillae_ are united, forming a single bone, which in a
ventral view is seen to meet the maxillae and vomero-palatines, and in
a dorsal view to send back a nasal process (fig. 21, A, 1) between the
nasals.

The _maxillae_ are large bones, each terminating in a point
posteriorly. A single row of teeth similar to those on the
vomero-palatines runs along the outer margin of the maxillae and
premaxillae.

The posterior part of the upper jaw forms a mass of cartilage which
extends forwards towards the maxillae as a long pointed process whose
ventral surface and sides are overlapped by a membrane bone, the
_pterygoid_ (fig. 21, 12).

The suspensorial bones include the =quadrate= and _squamosal_. The
=quadrate= (fig. 21, 15) which forms the true =suspensorium= is
directed forwards and outwards, and is terminated by a patch of
cartilage with which the mandible articulates.

The lower jaw or mandible remains partly cartilaginous, while its
ossifications include two membrane bones and one cartilage bone. The
cartilage bone is the =articular= (fig. 21, C, 18), it forms the
posterior part of the ramus, extends forwards for some distance along
its inner side, and is terminated posteriorly by a patch of cartilage
which articulates with the quadrate. The _dentary_ (fig. 21, C, 20) is
a large bone which forms the anterior part and nearly all the outer
half of each ramus, and bears teeth similar to those of the upper jaw.
Attached to its inner face is a long slender _splenial_ (fig. 21, C,
21).

THE HYOID APPARATUS.

This consists of the hyoid arch and part of the first two branchial
arches.

The =hyoid arch= (fig. 29, A, 2) consists of a pair of =cornua=, each
of which is divided into two halves. The dorsal half forming the
=cerato-hyal= is mainly ossified though tipped with cartilage, and is
connected by ligament with the suspensorium. The ventral half
(=hypo-hyal=) is cartilaginous, and is connected with the
basibranchial.

The =branchial arches= consist of a median piece, the =basibranchial=,
which is ossified in the centre and cartilaginous at either end, and
of two pairs of =cerato-branchials= which are attached to the
cartilaginous part (fig. 29, A, 8) of the basibranchial. The first
cerato-branchial is chiefly ossified, the second (fig. 29, A, 4) is a
good deal smaller and is cartilaginous. Both are united dorsally to a
single =epi-branchial=, which is terminated by a small cartilaginous
area at the free end but is elsewhere well ossified.

C. THE RIBS.

The ribs are short imperfectly ossified structures, bifid at their
proximal end where they articulate with the transverse processes, and
tipped both proximally and distally with cartilage. The dorsal portion
of the proximal end corresponds to the =tuberculum= of the ribs of
higher animals, and the ventral portion to the =capitulum=. Some of
the anterior ribs have a step-like notch on their dorsal surfaces.

The second to twelfth ribs are fairly equal in size, but further back
they decrease slightly. The ribs which connect the sacral vertebrae
with the ilia are however large. The short ribs borne on the anterior
caudal vertebrae are cartilaginous.

D. THE STERNUM.

The sternum (fig. 22, A, 6) is a rather broad plate of cartilage,
drawn out posteriorly into a median process marked by a prominent