trigeminal nerve pass out (fig. 39, V 1 & 2). It is hollowed out
posteriorly by the cavity in which the auditory organ lies, and its
inner wall as seen in longitudinal section is pierced by a foramen
through which the external carotid artery and facial nerve leave the
cranial cavity,--the nerve finally leaving the skull through a small
oval foramen on the anterior face of the pro-otic near its junction
with the quadrate.

Between the pro-otic and opisthotic as seen in a longitudinal section
of the skull is a large opening constricted in the middle. This is
the =internal auditory meatus= (fig. 39, VIII.). Through it the
auditory nerve leaves the cranial cavity and enters the ear. The ramus
vestibularis leaves through the dorsal part of the hole, the ramus
cochlearis through the ventral.

The cavity of the auditory or periotic capsule communicates with the
exterior by a fairly large hole, the =fenestra ovalis=, which lies
between the opisthotic and pro-otic, and opens into a deep depression,
the =tympanic cavity=, which is seen in a posterior view of the skull
lying just external to the exoccipital. The cavity communicates with
the exterior by a large opening, the =external auditory meatus= (fig.
38, 22).

Several other openings are seen in the tympanic cavity; through one at
the extreme posterior end the pneumogastric and spinal accessory
nerves leave the skull, and through another, a little further
forwards, the glossopharyngeal.

The auditory ossicles consist of a long bony =columella=, whose inner
end fits into the fenestra ovalis, while the outer end is attached to
a small cartilaginous plate, the =extra-columella=, which is united to
the tympanum.

The =Optic capsules=.

The skeletal structures developed in connection with the optic capsule
do not become united to the skull. They consist of:--

(_a_) the =sclerotic=, a cartilaginous sheath investing the eye and
bearing

(_b_) a ring of ten small bony scales.

There is no _lachrymal_ bone.

The =Olfactory= or =Nasal capsules=.

The basicranial axis in front of the basisphenoid remains
cartilaginous, neither presphenoid nor mesethmoid bones are developed,
and the orbits in a dry skull communicate by a wide space through
which the second, third, fourth, and sixth cranial nerves pass out.
Separate _nasal_ bones also do not occur, the large prefrontals
extending over the area usually occupied by both nasals and
lachrymals.

The only bone developed in connection with the nasal capsules is the
_vomer_ (fig. 39, 19), an unpaired bone lying ventral to the
mesethmoid cartilage, and in contact laterally with the maxillae,
premaxillae and palatines.

3. THE UPPER JAW AND SUSPENSORIAL APPARATUS.

A number of pairs of bones are developed in connection with the upper
jaw and suspensorial apparatus, one pair, the =quadrates=, being
cartilage bones, while the rest are all membrane bones.

The _squamosals_ (fig. 38, 2) are large bones which, lying external to
the auditory bones, extend dorsalwards to meet the parietals and
postfrontals, and form a large part of the false roof of the skull.
They are united ventrally with the quadrates and quadratojugals.

Each =quadrate= (fig. 38, 3) forms the outer boundary of the tympanic
cavity, and is firmly united on its inner side with the opisthotic,
exoccipital, and pterygoid. Dorsally it is fixed to the squamosal and
anteriorly to the quadratojugal. Its outer surface is marked by a deep
recess, and it ends below in a strong condyle with which the mandible
articulates. In front of the quadrates are a pair of thin plate-like
bones, the _quadratojugals_ which are united in front to the jugals or
malars.

The _jugals_ (fig. 38, 17) are also thin plate-like bones, and form
part of the posterior boundary of the orbit. They are attached
dorsally to the postfrontals, and anteriorly to the maxillae, while
each also sends inwards a horizontal process which meets the pterygoid
and palatine.

The _maxillae_ (figs. 38 and 39, 16) are a pair of large
vertically-placed bones, each drawn out ventrally into a straight,
sharp, cutting edge. They form the lateral boundaries of the anterior
nares, and each sends dorsalwards a process which meets the
postfrontal. Each also sends inwards a horizontal =palatine process=,
which meets the palatine and vomer, and also forms much of the floor
of the narial passage.

The _premaxillae_ (figs. 38 and 39, 15) are a pair of very small bones
forming the floor of the anterior narial opening, they are wedged in
between the two maxillae, and send back processes which meet the vomer
and palatines.

The _palatines_ (fig. 39, 10) are a pair of small bones firmly united
with the pterygoids behind, with the maxillae and jugals externally,
and with the vomer in the middle line. Each also gives off a palatine
plate which unites with the expanded lower edge of the vomer, and
forms the ventral boundary of the posterior nares. Anteriorly the
palatines form the posterior boundary of a large foramen through which
the ophthalmic branches of the fifth and seventh nerves pass to the
olfactory organs.

The _pterygoids_ (fig. 39, 9) are a pair of large bones which unite
with one another by a long median suture. They are united also with
the palatines in front, and with the quadrate, basisphenoid,
basi-occipital, and exoccipitals behind. Each also sends dorsalwards a
short =alisphenoid plate= which meets that from the parietal.

Piercing the posterior end of the _pterygoid_ is the prominent opening
of the carotid canal; a bristle passed into this hole emerges through
a foramen lying between the pro-otic and the alisphenoid process of
the pterygoid.

(2) THE LOWER JAW OR MANDIBLE.

The =mandible= consists of one unpaired bone, formed by the fusion of
the two _dentaries_, and five pairs of bones, called respectively the
=articular=, _angular_, _supra-angular_, _splenial_ and _coronoid_.

The fused _dentaries_ (fig. 38, 12) form by far the largest of the
bones; they constitute the flattened anterior part of the mandible,
and extend back below the other bones almost to the end of the jaw.

The _coronoid_ is the most anterior of the paired bones, it forms a
prominent process to which the muscles for closing the jaw are
attached.

The =articular= (fig. 38, 11) is expanded, and with the
_supra-angular_ forms the concave articulating surface for the
quadrate.

The _splenial_ (fig. 38, 10) is a thin plate applied to the inner
surface of the posterior part of the mandible.

The _angular_ (fig. 38, 13) is a slender plate of bone lying below the
supra-angular and splenial.

(3) THE HYOID.

The hyoid apparatus is well developed, parts of the first two
branchial arches being found, as well as of the hyoid proper. It
consists of a more or less oblong flattened =basilingual plate= or
=body of the hyoid= which represents the fused ventral ends of the
hyoid and branchial arches of the embryo, and is drawn out into a
point anteriorly. The greater part is formed of unossified cartilage,
but at the posterior end it is bilobed, and a pair of ossified tracts
occur. To its sides are attached three pairs of structures, which are
portions of the hyoid and first and second branchial arches
respectively.

The free part of the =hyoid= consists of a small piece of cartilage
attached to the anterior part of the basilingual plate at its widest
portion (fig. 53, 2).

The =anterior cornu= or free part of the =first branchial arch= is
much the largest of the three structures. Its proximal portion
adjoining the basilingual plate is cartilaginous, as is its distal
end; the main part is however ossified.

The =posterior cornu= or free part of the =second branchial arch=
(fig. 53, 4) consists of a short flattened cartilaginous bar arising
from the bilobed posterior end of the basilingual plate.

The hyoid apparatus has no skeletal connection with the rest of the
skull.

2. THE APPENDICULAR SKELETON.

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

THE PECTORAL GIRDLE.

The pectoral girdle has an anomalous position, being situated
internal or ventral to the ribs. It consists of three bones, a dorsal
bone, the =scapula=, an anterior ventral bone, the =precoracoid=, and
a posterior ventral bone, the =coracoid=.

The =scapula= is a small somewhat rod-shaped bone forming about
two-thirds of the glenoid cavity. At its proximal end it is closely
united with the precoracoid, the two bones ossifying continuously. It
tapers away distally, and is directed dorsalwards towards the
carapace.

The =precoracoid= forms an angle of about 130° with the scapula, with
which it is completely fused at its proximal end. Its distal end is
somewhat expanded and flattened, and is terminated by a
fibrocartilaginous =epiprecoracoid= which meets its fellow. It takes
no part in the formation of the glenoid cavity.

The =coracoid= is a large flattened blade-shaped bone forming about
one-third of the glenoid cavity. It does not meet its fellow in a
ventral symphysis, and is terminated by a cartilaginous =epicoracoid=.
The glenoid articulating surfaces of both scapula and coracoid are
lined by a thick pad of cartilage.

THE ANTERIOR LIMB.

This is divisible into three portions, the =upper arm=, =fore-arm= and
=manus=.

The =upper arm= contains a single bone, the =humerus=.

The =humerus= (fig. 40, A, 1) is a stout, nearly straight, somewhat
flattened bone widely expanded at both ends. At the proximal end is
the large hemispherical =head=, which articulates with the glenoid
cavity. Behind the head the bone is drawn out into another large
rounded process. Below the head the shaft bears a small outgrowth
which is continuous with a larger one on the flexor surface (see p.
29). The bone is terminated distally by the =trochlea=, consisting of
three partially distinct convex surfaces which articulate with the
bones of the fore-arm.

The =fore-arm= includes two bones, the =radius= and =ulna=; both these
are small bones, and are immovably fixed to one another proximally and
distally.

The =radius= or pre-axial bone is the larger of the two, and is a
rod-like bone terminated at either end by an epiphysis. It articulates
at its proximal end with the humerus, and at its distal end with the
radiale or scaphoid bone of the carpus.

The =ulna= (fig. 40, A, 3) or postaxial bone is shorter than the
radius, and more expanded at its proximal end, where it articulates
with the humerus. It articulates distally with the intermedium (lunar)
and the ulnare (cuneiform) bones of the carpus. All three bones of the
arm have their terminations formed by epiphyses which ossify from
centres distinct from those forming the shafts.

The =Manus= consists of the =carpus= or =wrist= and the =hand= which
includes the metacarpals and phalanges.

The =carpus= consists of a series of ten small bones, one of which,
the =pisiform= (fig. 40, A, 10), differs from the others in being
merely an ossification in the tendon of a muscle. The remaining nine
bones are arranged in a proximal row of three, the =ulnare= (fig. 40,
A, 6), =intermedium=, and =radiale=, and a distal row of five
(carpalia 1-5), each of which supports one of the metacarpals. A ninth
bone, the =centrale= (fig. 40, A, 7), is wedged in between the two
rows. The ulnare, intermedium and pisiform are comparatively large
flattened bones, the others are small and cubical.

The =hand=. This is composed of five digits, each of which consists of
a metacarpal and of a varying number of phalanges.

The =metacarpals=. The first metacarpal (fig. 40, A, 11) is a short
flattened bone, the others are all elongated and cylindrical, and are
terminated proximally by slightly concave surfaces, and distally by
slightly convex ones.

The =phalanges=. The first and fifth digits both have two phalanges,
the second, third, and fourth have each three. The distal phalanx of
the first digit is stout and curved, and bears a horny claw; those of
the other digits are flattened and more or less pointed.

[Illustration FIG. 40. A. ANTERIOR LIMB OF A YOUNG HAWKSBILL TURTLE
(_Chelone imbricata_) × 1/4 (Brit. Mus.). B. POSTERIOR LIMB OF A LARGE
GREEN TURTLE (_Chelone midas_) × 1/8 (Camb. Mus.).

    1. humerus.
    2. radius (almost hidden by the ulna).
    3. ulna.
    4. radiale.
    5. intermedium.
    6. ulnare.
    7. centrale.
    8. carpale I.
    9. carpale IV.
    10. pisiform.
    11. first metacarpal.
    12. femur.
    13. tibia.
    14. fibula.
    15. tibiale intermedium and centrale fused.
    16. fibulare.
    17. tarsale 1.
    18. tarsale 2.
    19. tarsalia 4 and 5 fused.
    20. first metatarsal.
    21. fifth metatarsal.
    I, II, III, IV, V, digits.]

THE PELVIC GIRDLE.

The pelvic girdle consists of three bones; a dorsal bone, the =ilium=,
an anterior ventral bone, the =pubis=, and a posterior ventral bone,
the =ischium=. All three bones contribute largely to the formation of
the =acetabulum=, with which the head of the femur articulates.

The =ilium= is a small slightly curved bone, which unites ventrally
with the pubis and ischium, and extends dorsalwards and backwards to
meet the distal ends of the sacral ribs.

The =pubis= is the largest bone of the three; its distal end forms a
wide bilobed plate, the inner lobe meeting its fellow in a median
symphysis, while the other lobe or lateral process extends outwards.
Attached to the symphysis in front is a cartilaginous =epipubis=,
while behind, the two pubes are terminated by a wide rounded
cartilaginous area.

The =ischium=, the smallest bone of the three, is flattened and like
the pubis meets its fellow in a median symphysis. A narrow band of
cartilage connects the symphysis pubis with the symphysis ischii, and
separates the two =obturator foramina= from one another.

THE POSTERIOR LIMB.

This is divisible into three portions, the =thigh=, the =crus= or
=shin=, and the =pes=.

The =thigh= includes a single bone, the =femur=.

The =femur= (fig. 40, B, 12) is a short thick bone, with a prominent
rounded =head= articulating with the acetabulum. Behind this head is a
deep pit, beyond which is a roughened area corresponding with the
great trochanter of mammals. The distal end is expanded and somewhat
convex.

The bones of the =crus= or =shin= are the =tibia= and =fibula=. These
are both straight rod-like bones with expanded terminations which
closely approach one another, while elsewhere the bones diverge
considerably.

The terminations of all three of the leg bones are formed by
epiphyses.

The =Pes= consists of the =tarsus= or =ankle=, and the =foot=, which
is made up of five digits.

The =tarsus=. The tarsal bones of the Turtle do not retain their
primitive arrangement to such an extent as do the carpals. They are
arranged in a proximal row of two and a distal row of four. Of the
bones in the proximal row the postaxial one is much the smaller and is
the =fibulare=; the larger pre-axial one (fig. 40, B, 15) represents
the =tibiale=, =intermedium=, and =centrale= fused, and articulates
with both tibia and fibula. The first three distal tarsalia are all
small bones and are very similar in size, and each articulates
regularly with the corresponding metatarsal. The fourth bone (fig. 40,
B, 19) is much larger, and represents tarsalia 4 and 5 fused. The
first two distal tarsalia articulate with the pre-axial tarsal of the
proximal row, the third only with its neighbours the second, and the
fused fourth and fifth. The latter articulates with both bones of the
proximal row.

Each =digit= consists of a metatarsal and of a varying number of
phalanges.

The =metatarsals=. The first metatarsal (fig. 40, B, 20) is broad and
flattened, the second, third and fourth, are all elongated bones with
nearly flat terminations formed by small epiphyses. The fifth is large
and flattened, and the articular surface for the phalanx is situated
somewhat laterally.

The =phalanges=. The first digit has two phalanges and is the stoutest
of them all; its distal phalanx is sheathed in a large horny claw. The
other digits, of which the third is the longest, have each three
phalanges. The distal phalanges of the second and third digits are
flattened and pointed and bear small horny claws.

FOOTNOTES:

[89] Another view commonly held is that the neural and costal plates
are respectively formed by the expanded neural arches and ribs.

CHAPTER XV.

THE SKELETON OF THE CROCODILE.

The species chosen for description is _C. palustris_, a form occurring
throughout the Oriental region, but the description would apply almost
equally well to any of the other species of the genus _Crocodilus_,
and with comparatively unimportant modifications to any of the living
Crocodilia.

I. EXOSKELETON.

The exoskeleton of the Crocodile is strongly developed and includes
elements of both epidermal and dermal origin.

_a._ The =epidermal exoskeleton= is formed of a number of horny
=scales= or plates of variable size covering the whole surface of the
body. Those covering the dorsal and ventral surfaces are oblong in
shape, and are arranged in regular rows running transversely across
the body. The scales covering the limbs and head are mostly smaller
and less regularly arranged, and are frequently raised into a more or
less obvious keel. Those covering the dorsal surface of the tail are
very prominently keeled.

The epidermal exoskeleton also includes the horny =claws= borne by the
first three digits of both manus and pes.

_b._ The =dermal exoskeleton=. This has the form of bony =scutes=
which underlie the epidermal scales along the dorsal surface of the
trunk and anterior part of the tail. Except in very young individuals
the epidermal scales are rubbed off from these scutes, which
consequently come to project freely on the surface of the body. Each
scute is a nearly square bony plate, deeply pitted or sculptured, and
marked by a strong ridge on its dorsal surface, while its ventral
surface is smooth. Contiguous scutes are united to one another by
interlocking sutures.

The scutes are arranged in two distinct areas, viz. (1) a small
anterior =nuchal shield= which lies just behind the head and is formed
of six large scutes more or less firmly united together, and (2) a
larger posterior =dorsal shield= covering the whole of the back and
anterior part of the tail, and formed of smaller scutes, which are
arranged in regular transverse rows, and progressively diminish in
size when followed back.

The =teeth= are exoskeletal structures, partly of dermal, partly of
epidermal origin. They lie along the margins of the jaws and are
confined to the premaxillae, maxillae and dentaries. They are simple
conical structures, without roots; each is in the adult placed in a
separate socket, and is replaced by another which as it grows comes to
occupy the pulp cavity of its predecessor. In the young animal the
teeth are not placed in separate sockets but in a continuous groove.
This feature is met with also in the Ichthyosauria. The groove
gradually becomes converted into a series of sockets by the ingrowth
of transverse bars of bone. The anterior teeth are sharply pointed and
slightly recurved, the posterior ones are more blunt.

The upper jaw bears about nineteen pairs of teeth, the lower jaw about
fifteen pairs. The largest tooth in the upper jaw is the tenth, and in
the lower jaw the fourth.

The three living families of Crocodilia, the Crocodiles, Alligators
and Garials, can be readily distinguished by the characters of the
first and fourth lower teeth. In Alligators both first and fourth
lower teeth bite into pits in the upper jaw; in Garials they both bite
into notches or grooves in the upper jaw. In Crocodiles the first
tooth bites into a pit, the fourth into a notch in the upper jaw.

II. ENDOSKELETON.

1. THE AXIAL SKELETON.

This includes the vertebral column, the skull, and the ribs and
sternum.

A. THE VERTEBRAL COLUMN.

The vertebral column is very long, consisting of some sixty vertebrae.
It can be divided into the usual five regions, the cervical, thoracic,
lumbar, sacral, and caudal regions.

[Illustration FIG. 41. FIRST FOUR CERVICAL VERTEBRAE OF A CROCODILE
(_C. vulgaris_). (Partly after VON ZITTEL.)

    1. pro-atlas.
    2. lateral portion of atlas.
    3. odontoid process.
    4. ventral portion of atlas.
    5. neural spine of axis.
    6. postzygapophysis of fourth vertebra.
    7. tubercular portion of fourth cervical rib.
    8. first cervical rib.
    9. second cervical rib.
    10. convex posterior surface of centrum of fourth vertebra.]

THE CERVICAL VERTEBRAE.

Counting as cervical all those vertebrae which are anterior to the
first one whose ribs meet the sternum, there are nine cervical
vertebrae, all of which bear ribs.

As a type of the cervical vertebrae the fifth may be taken. It has a
short cylindrical =centrum= deeply concave in front and convex behind.
From the anterior part of the ventral surface of the centrum arises a
short =hypapophysis=, and on each side is a facet with which the lower
limb (=capitulum=) of the cervical rib articulates. The =neural arch=
is strongly developed and drawn out dorsally into a long =neural
spine=, in front of which are a pair of upstanding processes bearing
the prominent upwardly and inwardly directed =prezygapophyses=. At the
sides and slightly behind the neural spine are a corresponding pair of
processes bearing the =postzygapophyses=, which look downwards and
outwards. At the point where it joins the centrum the neural arch is
drawn out into a short blunt =transverse process= with which the upper
limb (=tuberculum=) of the cervical rib articulates. The sides of the
neural arch are slightly notched behind for the exit of the spinal
nerves.

The first or =atlas= vertebra differs much from any of the others, and
consists of four quite detached portions, a ventral arch, with two
lateral portions and one dorsal. The =ventral arch= (fig. 41, 4) is
flat below and slightly concave in front, forming together with two
flattened surfaces on the lateral portions a large articulating
surface for the occipital condyle of the skull. Its posterior face is
bevelled off and forms with a second pair of facets on the lateral
portions a surface with which the odontoid process of the second
vertebra articulates. The postero-lateral surfaces of the ventral arch
also bear a pair of little facets with which the cervical ribs
articulate. The lateral portions are somewhat flattened and expanded,
and bear in addition to those previously mentioned a pair of small
downwardly directed facets, the postzygapophyses, which articulate
with the prezygapophyses of the second vertebra. The dorsal portion
(fig. 41, 1) is somewhat triangular in shape, and overhangs the
occipital condyle. It is often regarded as the neural arch of a
vertebra in front of the atlas and is called the _pro-atlas_; but as
it is a membrane bone it is not properly a vertebral element.

The second or =axis vertebra= also differs a good deal from the other
cervicals. The centrum is massive, and is terminated in front by a
very large slightly concave articulating surface formed by the
=odontoid process= (fig. 41, 3) which is united with the centrum by
suture only, and is really the detached centrum of the first vertebra.
The cervical rib (fig. 41, 9) articulates with two little
irregularities on the odontoid process. The posterior surface of the
centrum is convex. The neural arch is strongly developed and
terminated dorsally by a long neural spine (fig. 41, 5), its sides are
notched, slightly in front and more prominently behind for the exit of
the spinal nerves. It is drawn out in front into two little processes
bearing a pair of upwardly and outwardly directed prezygapophyses,
while the postzygapophyses are similar to those of the other cervical
vertebrae.

The last two cervical vertebrae resemble the succeeding thoracic
vertebrae, in the increased length of the transverse processes, and
the shifting dorsalwards of the facet with which the capitulum of the
rib articulates.

THE THORACIC VERTEBRAE.

The thoracic vertebrae commence with the first of those that bears
ribs reaching the sternum. They are ten in number, and the first eight
are directly connected with the sternum by ribs.

The =third= of them may be taken as a type. It has a thick cylindrical
centrum, concave in front and convex behind, there is a slight
hypapophysis, and the centrum is suturally united with a strong neural
arch enclosing a narrow neural canal. The neural arch is drawn out
dorsally into a wide truncated neural spine, and laterally into two
prominent transverse processes, with the ends of which the tubercula
of the ribs articulate, while the capitulum articulates in each case
with a step-like facet (fig. 42, A, 3) on the anterior face of the
transverse process. The prezygapophyses (fig. 42, A, 2) are borne on
outgrowths from the bases of the transverse processes, and the
postzygapophyses on outgrowths at the base of the neural spine.

The thoracic vertebrae behind the third have no hypapophyses, and the
capitular facets gradually come to be placed nearer and nearer the
ends of the transverse processes, at the same time becoming less
prominent; otherwise these vertebrae are just like the third.

[Illustration FIG. 42. ANTERIOR VIEW OF A, A LATE THORACIC AND B, THE
FIRST SACRAL VERTEBRA OF A YOUNG CROCODILE (_C. palustris_). × 1/3.

    1. neural spine.
    2. process bearing prezygapophysis.
    3. facet for articulation with the capitulum of the rib.
    4. sacral rib.
    5. surface which is united with the ilium.
    6. concave anterior face of centrum.]

In the first and second thoracic vertebrae the capitulum of the rib
articulates, not with a facet on the transverse process, but with a
little elevation borne at the line of junction of the centrum and
neural arch.

THE LUMBAR VERTEBRAE.

These are five in number, and are precisely like the posterior
thoracic vertebrae, except in the fact that the transverse processes
have no facets for the articulation of ribs.

THE SACRAL VERTEBRAE.

These are two in number, and while the centrum of the first is concave
in front (fig. 42, B, 6) and nearly flat behind, that of the second is
flat in front and concave behind. Each has a pair of strong =ribs=
(fig. 42, B, 4) firmly ankylosed in the adult with a wide surface
furnished partly by the centrum, partly by the neural arch. The distal
ends of these ribs are united with the ilia. The character of the
neural spines and zygapophyses is the same as in the thoracic
vertebrae.

THE CAUDAL VERTEBRAE.

These are very numerous, about thirty-four in number. The first
differs from all the other vertebrae of the body in having a biconvex
centrum. The succeeding ones are procoelous and are very much like the
posterior thoracic and lumbar vertebrae, having high neural spines and
prominent straight transverse processes. They differ however in having
the neural spines less strongly truncated above, and the transverse
processes arise from the centra and not from the neural arches. When
followed further back the centra and neural spines gradually lengthen
while the transverse processes become reduced, and after the twelfth
vertebra disappear. Further back still the neural spines and
zygapophyses gradually become reduced and disappear, as finally the
neural arch does also, so that the last few vertebrae consist simply
of cylindrical centra.

Each caudal vertebra, except the first and the last eleven or so, has
a =V=-shaped =chevron bone= attached to the postero-ventral edge of
its centrum. The anterior ones are the largest and they gradually
decrease in size till they disappear.

B. THE SKULL[90].

The skull of the Crocodile is a massive depressed structure
presenting a number of striking characteristics, some of the more
important of which are:--

1. All the bones except the mandible, hyoid, and columella are firmly
united by interlocking sutures. In spite of this, however, growth of
the whole skull and of the component bones goes on continuously
throughout life, this growth being especially marked in the case of
the facial as opposed to the cranial part of the skull.

2. All the bones appearing on the dorsal surface are remarkable for
their curious roughened and pitted character; this feature is
prominent also in many Labyrinthodonts.

3. The size of the jaws and teeth is very great.

4. The mandibular condyle is carried back to some distance behind the
occipital condyle.

5. The occipital plane (see p. 386) of the skull is vertical.

6. The length of the secondary palate is remarkably great, and the
vomer takes no part in its formation.

7. The posterior nares are placed very far back, the nasal passages
being as in mammals separated from the mouth by the long secondary
palate.

8. There is a complicated system of Eustachian passages communicating
at one end with the tympanic cavity and at the other end with the
mouth cavity.

9. The interorbital septum is mainly cartilaginous, the presphenoidal
and orbitosphenoidal regions remaining unossified.

The =skull= is divisible into three parts:--

(1) the cranium, (2) the lower jaw, (3) the hyoid.

The =cranium= may again for purposes of description be divided into:--

1. the cranium proper or brain case;

2. the bones connected with the several special sense organs;

3. the bones of the upper jaw, and suspensorial apparatus.

1. THE CRANIUM PROPER OR BRAIN CASE.

[Illustration FIG. 43. PALATAL ASPECT A, OF THE CRANIUM, B, OF THE
MANDIBLE OF AN ALLIGATOR (_Caiman latirostris_). × 1/3. (Brit. Mus.)

    1. premaxillae.
    2. maxillae.
    3. palatine.
    4. pterygoid.
    5. posterior nares.
    6. transpalatine.
    7. posterior palatine vacuity.
    8. anterior palatine vacuity.
    9. basi-occipital.
    10. opening of median Eustachian canal.
    11. jugal.
    12. quadratojugal.
    13. quadrate.
    14. dentary.
    15. splenial.
    16. coronoid.
    17. supra-angular.
    18. angular.
    19. articular.
    20. lateral temporal fossa.
    21. openings of vascular canals leading into alveolar sinus.]

The cartilage and membrane bones of the cranium proper when taken
together can in most vertebrates be seen to be more or less arranged
in three rings or segments called respectively the =occipital=,
=parietal= and =frontal= segments; in the Crocodile however only the
occipital and parietal segments are clearly seen.

The =occipital segment= consists of four cartilage bones, three of
which together surround the =foramen magnum=.

The most ventral of these, the =basi-occipital= (figs. 43 and 45, 9),
forms the single convex =occipital condyle= for articulation with the
atlas, bounds the base of the foramen magnum, and is continuous
laterally with two larger bones, the =exoccipitals= (fig. 45, 24),
which meet one another dorsally and form the remainder of the boundary
of the foramen magnum. Each is drawn out externally into a strong
process, which is united below with the quadrate, and above with the
squamosal by a surface seen in a disarticulated skull to be very rough
and splintered. In a longitudinal section the anterior face of the
exoccipital is seen to be closely united with the opisthotic.

The exoccipital is pierced by a number of foramina, four lying on the
posterior surface. Just external to the foramen magnum is a small
foramen for the exit of the hypoglossal nerve (figs. 44 and 45, XII).
External to this is the foramen for the pneumogastric (fig. 44, X),
while more ventrally still is the foramen (fig. 44, 15) through which
the internal carotid artery enters the skull. Some distance further to
the side, and more dorsally, is a larger foramen which gives passage
to the facial nerve and certain blood-vessels.

In a median longitudinal section of the skull the hypoglossal foramen
is seen, and just in front of it a small foramen for a vein. Further
forwards the long slit-like opening between the exoccipital and
opisthotic is the =internal auditory meatus= (fig. 45, VIII) through
which the auditory nerve leaves the cranial cavity and enters the
internal ear.

The =supra-occipital= (fig. 45, 5) is a small bone which takes no part
in the formation of the foramen magnum, and is closely united in front
with the epi-otic. It is characteristic of Crocodiles that all the
bones of the occipital segment have their longer axes placed
vertically, and that they scarcely if at all appear on the dorsal
surface.

In front of the occipital segment is the =parietal segment=. The
dorsal and ventral portions of the two segments are in contact with
one another, but the lateral portions are widely separated by the
interposition of the =auditory= and =suspensorial bones=.

The =basisphenoid= (fig. 45, 12) is an unpaired wedge-shaped bone,
united along a deep vertical suture with the basi-occipital. The two
bones are, however, partially separated in the mid-ventral line by a
foramen, the opening of the =median Eustachian canal=, which leads
into a complicated system of Eustachian passages ultimately
communicating with the tympanic cavity.

The dorsal surface of the basisphenoid is well seen in a section of
the skull, but owing to the way it tapers ventrally, it appears on the
ventral surface only as a very narrow strip of bone wedged in between
the basi-occipital and pterygoids. In a lateral view it is seen to be
drawn out in front into an abruptly truncated process, the =rostrum=,
which forms part of the =interorbital septum.= On the anterior part of
the dorsal surface is a deep pit, the =pituitary fossa= or =sella
turcica=, at the base of which are a pair of foramina, through which
the carotid arteries pass. Dorsolaterally the basisphenoid articulates
with the =alisphenoids=.

[Illustration FIG. 44. LATERAL VIEW OF THE SKULL OF AN ALLIGATOR
(_Caiman latirostris_). × 1/3. (Brit. Mus.)

    1. premaxillae.
    2. maxillae.
    3. lachrymal.
    4. prefrontal.
    5. jugal.
    6. postfrontal.
    7. squamosal.
    8. quadrate.
    9. palatine.
    10. pterygoid.
    11. transpalatine.
    12. quadratojugal.
    13. exoccipital.
    14. basi-occipital.
    15. foramen by which carotid artery enters skull.
    16. external auditory meatus.
    17. frontal.
    18. supra-angular.
    19. articular.
    20. dentary.
    21. coronoid.
    22. angular.
    III, VI, opening for exit of oculomotor and abducens nerves.
    V, foramen ovale.
    X, pneumogastric foramen.
    XII, hypoglossal foramen.]

The =alisphenoids= (fig. 45, 13) are a pair of irregular bones which
arise from the basisphenoid antero-laterally, and are united dorsally
with the parietal, frontal, and postfrontals. They bound most of the
anterior part of the brain case, and each presents on its inner face a
deep concavity which lodges the cerebral hemisphere of its side.
Viewed from the ventral side the two alisphenoids are seen to almost
or quite meet one another immediately below the frontal, and then to
diverge, forming an irregular opening--partially closed by cartilage
in the fresh specimen,--through which the optic nerves leave the
cranial cavity. Further back the alisphenoids meet one another for a
narrow area, and then diverge again, so that between each and the
rostrum of the basisphenoid there appears an opening (fig. 44, III,
VI) through which the oculomotor and abducens nerves leave the
cranium. Further back still each is united for a short space with the
basisphenoid, pterygoid and quadrate, and then becomes separated from
the quadrate by a large foramen, the =foramen ovale= (fig. 44, V),
through which the whole of the trigeminal nerve passes out.

The dorsal portion of the parietal segment is formed by the _parietal_
(fig. 45, 4), which though double in the embryo, early comes to form a
single bone. It extends over the posterior part of the cranial cavity,
and is continuous in front with the frontal, behind with the
supra-occipital, and laterally with the postfrontals, squamosals,
alisphenoids, pro-otics and epi-otics. It forms the inner boundary of
a large rounded vacuity on the roof of the skull, the =supratemporal
fossa=.

The =frontal segment= is very imperfectly ossified, there being no
certain representatives of either the ventral member, the presphenoid,
or the lateral members, the orbitosphenoids. On the dorsal side there
is, however, a large development of membrane bones. There is a large
_frontal_ (fig. 45, 3), unpaired, except in the embryo, united behind
with the parietal and postfrontal, and drawn out in front into a long
process which is overlapped by the prefrontals and posterior part of
the nasals. The frontal ends off freely below, owing to the
orbitosphenoidal region being unossified, it forms a considerable part
of the roof of the cranial cavity, but takes no part in the formation
of the wall.

Each _prefrontal_ (fig. 45, 14) forms part of the inner wall of the
orbit and sends ventralwards a process which meets the palatine.

The _postfrontals_ (fig. 44, 6) are small bones lying at the sides of
the posterior part of the frontal. Each is united with a number of
bones, on its inner side with the frontal and parietal, behind with
the squamosal, and ventrally with the alisphenoid. It also unites by
means of a strong descending process with an upgrowth from the jugal,
and thus forms a =postorbital bar= separating the orbit from the
lateral temporal fossa. The postfrontal forms also part of the outer
boundary of the supratemporal fossa.

2. THE SENSE CAPSULES.

Skeletal capsules occur in connection with each of the three special
sense organs of sight, of hearing and of smell.

The =Auditory capsules= and associated bones.

Three bones, the =epi-otic=, =opisthotic= and =pro-otic=, together
form the auditory or =periotic= capsule of each side. They are wedged
in between the lateral portions of the occipital and parietal segments
and complete the cranial wall in this region. Their relations to the
surrounding structures are very complicated, and many points can be
made out only in sections of the skull passing right through the
periotic capsule. The relative position of the three bones is,
however, well seen in a median longitudinal section. The =opisthotic=
early becomes united with the exoccipital, while the =epi-otic=
similarly becomes united with the supra-occipital, the =pro-otic=
(fig. 45, 7),--seen in longitudinal section to be pierced by the
prominent =trigeminal foramen=--alone remaining distinct throughout
life. The three bones together surround the essential organ of hearing
which communicates laterally with the deep tympanic cavity by the
=fenestra ovalis=.

The =tympanic cavity=, leading to the exterior by the =external
auditory meatus= (fig. 44, 16), is well seen in a side-view of the
skull; it is bounded on its inner side by the periotic bones,
posteriorly in part by the exoccipital, and elsewhere mainly by the
quadrate. A large number of canals and passages open into it. On its
inner side opening ventro-anteriorly is the =fenestra ovalis=, opening
ventro-posteriorly the =internal auditory meatus= (fig. 45, VIII),
while dorsally there is a wide opening which forms a communication
through the roof of the brain-case with the tympanic cavity of the
other side. On its posterior wall is the prominent foramen through
which the facial nerve passes on its way to its final exit from the
skull through the exoccipital, this foramen is bounded by the
quadrate, squamosal, and exoccipital.

The opening of the fenestra ovalis is in the fresh skull occupied by
the expanded end of the auditory ossicle, the =columella=, whose outer
end articulates by a concave facet with a trifid =extra-columellar=
cartilage which reaches the tympanic membrane. The lower process of
this extra-columella passes into a cartilaginous rod which lies in a
canal in the quadrate and is during life continuous with Meckel's
cartilage within the articular bone of the mandible.

The columella and extra-columella are together homologous with the
chain of mammalian auditory ossicles.

The =Optic capsules= and associated bones.

Two pairs of bones are associated with the optic capsules, viz. the
_lachrymals_ and the _supra-orbitals_. The _lachrymal_ (fig. 44, 3) is
a fairly large flattened bone lying wedged in between the maxillae,
nasal, jugal, and prefrontal. It forms a considerable part of the
anterior boundary of the orbit, and is pierced by two foramina. On the
orbital edge is a large hole leading into a cavity within the bone
which lodges the naso-lachrymal sac, and communicates with the narial
passage by a wide second foramen near the anterior end of the bone.
The _supra-orbital_ is a very small loose bone lying in the eyelid
close to the junction of the frontal and prefrontal.

The =Olfactory capsules= and associated bones.

Two pairs of membrane bones, the _vomers_ and _nasals_, are developed
in association with the olfactory organ, but the =mesethmoid= is not
ossified.

The _vomers_ form a pair of delicate bones, each consisting of a
vertical plate (fig. 45, 15), which with its fellow separates the two
narial passages, and of a horizontal plate which forms much of their
roof. The vomers articulate with one another and with the pterygoids,
palatines, and maxillae.

The _nasals_ (fig. 45, 2) are very long narrow bones extending along
the middle line from the frontal almost to the anterior nares. They
are continuous laterally with the premaxillae, maxillae, lachrymals and
prefrontals. They form the roof of the narial passages.

3. THE UPPER JAW AND SUSPENSORIAL APPARATUS.

These are enormously developed in the Crocodile and are firmly united
to the cranium. It will be most convenient to begin by describing the
bones at the anterior end of the jaw and to work back thence towards
the brain-case. The most anterior bones are the _premaxillae_. The
_premaxillae_ (figs. 44 and 45, 1) are small bones, each bearing five
pairs of teeth, set in separate sockets in their alveolar borders.
They constitute almost the whole of the boundary of the =anterior
nares=, which are confluent with one another and form a large
semicircular opening in the roof of the skull, leading into the wide
narial passage. They are also partially separated from one another in
the ventral middle line, by the small =anterior palatine vacuity=
(fig. 43, A, 8). They form the anterior part of the broad =palate=.
The alveolar border on each side between certain of the teeth is
marked by pits which receive the points of the teeth of the other jaw.
The first pair of these pits in the premaxillae are often so deep as to
be converted into perforations. Pits of the same character occur
between the maxillary and mandibular teeth.

[Illustration FIG. 45. LONGITUDINAL SECTION THROUGH THE SKULL OF AN
ALLIGATOR (_Caiman latirostris_). × 1/3. (Brit. Mus.)

    1. premaxilla.
    2. nasal.
    3. frontal.
    4. parietal.
    5. supra-occipital.
    6. epi-otic.
    7. pro-otic.
      immediately in front of the figure 7 is the prominent foramen
      for the trigeminal nerve.
    8. opisthotic.
    9. basi-occipital.
    10. quadrate.
    11. pterygoid.
    12. basisphenoid.
    13. alisphenoid.
    14. prefrontal.
    15. vomer.
    16. maxilla.
    17. palatine.
    18. dentary.
    19. splenial.
    20. angular.
    21. supra-angular.
    22. articular.
    23. coronoid.
    24. exoccipital.
    25. squamosal.
    26. jugal.
    27. external mandibular foramen.
    28. internal mandibular foramen.
    VIII. internal auditory meatus.
    XII. hypoglossal foramen.]

The _maxillae_ (figs. 43, A, 2 and 44, 2) are a pair of very large
bones and bear the remaining teeth of the upper jaw, set in sockets
along their alveolar borders. On the dorsal side each maxillae is
continuous with the premaxillae, nasal, lachrymal, and jugal, while
ventrally it meets its fellow in a long straight suture and forms the
greater part of the long bony palate. The maxillae are separated in the
middle line posteriorly by processes from the palatines, while further
back they meet the transpalatines. The internal or nasal surface, like
that of the premaxillae, is excavated by a deep longitudinal groove,
the =narial passage=. In a ventral view of the skull a number of small
openings (fig. 43, A, 21) are seen close to the alveolar border, these
are the openings of small vascular canals which lead into the
=alveolar sinus=, a passage traversing the maxillae, and transmitting
the superior maxillary branch of the trigeminal nerve and certain
blood-vessels. This alveolar sinus opens posteriorly by the more
external of the two large holes in the maxillae, which lie close to the
anterior edge of the posterior palatine vacuity, to be described
immediately. The more internal of these holes, on the other hand,
leads into a cavity lodging the nasal sac. Behind the maxillae the
completeness of the palate is broken up by the large oval =posterior
palatine vacuities= (fig. 43, A, 7); these are separated from one
another in the middle line by the palatines, and are bounded elsewhere
by the maxillae, transpalatines, and pterygoids.

The _palatines_ (fig. 43, A, 3) are long and rather narrow bones
interposed between the maxillae in front and pterygoids behind. They
meet one another in a long suture and form much of the posterior part
of the palate, while the whole length of their dorsal surface
contributes to the floor of the narial passage. The dorsal surface of
each bone is also drawn out on its outer side into a prominent ridge
which forms much of the side and roof of the narial passage, being in
contact with the vomer and pterygoid, and at one point by means of a
short ascending process with the descending process of the
prefrontal.

The _pterygoids_ (figs. 43, A, 4, and 45, 11) are a pair of large
bones, each consisting of a median more or less vertical part, which
becomes ankylosed to its fellow in the middle line early in life, and
of a wide horizontal part which meets the transpalatine. They
completely surround the posterior nares (fig. 43, A, 5) and their
median portions form the whole boundary of the posterior part of the
narial passage, and assist the palatines and vomers in bounding the
middle part. The horizontal parts form the posterior part of the
secondary palate, while the dorsal surface of each looks into the
=pterygoid fossa=, a large cavity lying below the quadrate and
quadratojugal at the side of the skull. The lateral margin adjoining
the transpalatine is in the fresh skull terminated by a plate of
cartilage against which the mandible plays. Dorsally the pterygoid
articulates with the basisphenoid, quadrate, and alisphenoid.

The _transpalatines_ (fig. 44, 11) connect the pterygoids with the
jugals and maxillae, articulating with each of the three bones by a
long pointed process. The jugal process meets also a down-growth from
the postfrontal.

The _jugals_ or _malars_ (fig. 44, 5) are long somewhat flattened
bones which are united to the lachrymals and maxillae in front, while
passing backwards each is united behind to the _quadratojugal_ (fig.
44, 12), the two forming the =infratemporal arcade= which constitutes
the external boundary of the orbit and lateral temporal fossa. The
jugal is united below to the transpalatine, and the two bones together
form an outgrowth, which meeting that from the postfrontal forms the
=postorbital bar=, and separates the orbit from the lateral temporal
fossa. The quadratojugals are small bones and are united behind with
the quadrates.

The =quadrate= (figs. 43, A, 13 and 44, 8) of each side is a large
somewhat flattened bone firmly fixed in among the other bones of the
skull. It is terminated posteriorly by an elongated slightly convex
surface, coated with cartilage in the fresh skull, by which the
mandible articulates with the cranium. The dorsal surface of the
quadrate is flat behind, further forwards it becomes much roughened
and articulates with the exoccipital and squamosal; further forwards
still it becomes marked by a deep groove which forms the floor of the
external auditory meatus and part of the tympanic cavity. The anterior
boundary of the quadrate is extremely irregular, it is united dorsally
with the postfrontal, pro-otic, and squamosal, and more ventrally with
the alisphenoid. The smooth ventral surface looks into the pterygoid
fossa. In front the quadrate forms the posterior boundary of the
supratemporal fossa and foramen ovale, and is continuous with the
alisphenoid, while it sends down a thin plate meeting the pterygoid
and basisphenoid. On the inner side of the dorsal surface of the
quadrate near the condyle, is a small foramen which leads into a tube
communicating with the tympanic cavity, by a foramen lying in front of
and ventral to that for the exit of the facial nerve. By this tube air
can pass from the tympanic cavity into the articular bone of the
mandible.

The _squamosal_ (fig. 44, 7) meets the quadrate and exoccipital below,
and forms part of the roof of the external auditory meatus, while
above it forms part of the roof of the skull and has a pitted
structure like that of the other bones of the roof. It is continuous
with the postfrontal in front, forming with it the =supratemporal
arcade= which constitutes the outer boundary of the supratemporal
fossa. It meets also the parietal on its inner side, forming the
=post-temporal bar=, the posterior boundary of the supratemporal
fossa.

It may be useful to recapitulate the large vacuities in the surface of
the Crocodile's cranium.

_Dorsal surface._

1. =The Supratemporal fossae=. Each is bounded internally by the
parietal, behind by the =post-temporal bar= formed by the parietal and
squamosal, and externally by the =supratemporal arcade= formed by the
squamosal and postfrontal. The postfrontal meets the parietal in front
and forms the anterior boundary of the supratemporal fossa.

2. The =Lateral temporal= or =infratemporal fossae=. These lie below
and to the outer side of the supratemporal fossae. Each is bounded
dorso-internally by the supratemporal arcade; and behind by a
continuation of the post-temporal bar formed by the quadrate and
quadratojugal. The external boundary is the =infratemporal arcade=
formed of the quadratojugal and jugal, while in front the fossa is
separated from the orbit by the =postorbital bar= formed by the
junction of outgrowths from the postfrontal and jugal.

3. The =Orbits=. Each is bounded behind by the postorbital bar,
externally by the jugal forming a continuation of the infratemporal
arcade, in front by the lachrymal, and internally by the frontal and
prefrontal.

4. The =Anterior nares=. These form an unpaired opening bounded by the
premaxillae.

_Posterior surface._

5. The =Foramen magnum=. The exoccipitals form the chief part of its
boundary, but part of the ventral boundary is formed by the
basi-occipital.

6. The =Pterygoid fossae=. These form a pair of large cavities at the
sides of the occipital region of the skull. The dorsal boundary is
formed by the quadrate and quadratojugal, the ventral by the
pterygoid, the internal chiefly by the quadrate, pterygoid,
alisphenoid, and basisphenoid. The transpalatine forms a small part of
the external boundary which is incomplete.

_Ventral surface._

7. The =Posterior nares=. These form a median unpaired opening (fig.
43, A, 5) bounded by the pterygoids.

8. The =Posterior palatine vacuities=. Each is bounded by the maxillae
in front, the maxillae and transpalatine externally, the transpalatine
and pterygoid behind, and the palatine on the inner side (fig. 43, A,
7).

9. The =Anterior palatine vacuity=. This is unpaired and is bounded by
the premaxillae (fig. 43, A, 8).

(_b_) THE LOWER JAW OR MANDIBLE.

The mandible is a strong compact bony structure formed of two halves
or =rami=, which are suturally united at the symphysis in the middle
line in front. Each ramus is formed of six separate bones.

The most anterior and largest of these is the _dentary_ (figs. 44, 20,
and 45, 18), which forms the symphysis, and greater part of the
anterior half of the jaw, and bears along the outer part of its dorsal
border a number of sockets or =alveoli= in which the teeth are placed.
Lying along the inner side of the dentary is a large splint-like bone,
the _splenial_ (fig. 45, 19), which does not extend so far forwards as
the symphysis, and is separated from the dentary posteriorly by a
large cavity. Forming the lower part of all the posterior half of the
jaw is the large _angular_ (figs. 44, 22, and 45, 20), which underlies
the posterior part of the dentary in front and sends a long process
below that bone to the splenial. On the inner side of the jaw there is
an oval vacuity, the =internal mandibular foramen= (fig. 45, 28),
between the angular and the splenial; through this pass blood-vessels
and branches of the inferior dental nerve. Lying dorsal to the angular
is another large bone, the _supra-angular_ (figs. 44, 18, and 45, 21).
It extends back as far as the posterior end of the jaw and forwards
for some distance dorsal to the dentary and splenial. It forms part of
the posterior margin of a large vacuity, the =external mandibular
foramen=, which is bordered above and in front by the dentary and
below by the angular; it gives passage to the cutaneous branch of the
inferior dental nerve. The concave surface for articulation with the
mandible and much of the posterior end of the jaw is formed by a short
but solid bone, the =articular= (fig. 45, 22), which in young skulls
rather readily becomes detached. The remaining mandibular bone is the
_coronoid_ (fig. 45, 23), a very small bone of irregular shape
attached to the angular below, and to the supra-angular and splenial
above.

(_c_) THE HYOID.

The hyoid of the Crocodile consists of a wide flattened plate of
cartilage, the =basilingual plate= or =body of the hyoid=, and a pair
of =cornua=.

The =basilingual plate= (fig. 53, 1) is rounded anteriorly and marked
by a deep notch posteriorly. The =cornua= (fig. 53, 3), which are
attached at a pair of notches near the middle of the outer border of
the basilingual plate, are partly ossified, but their expanded ends
are formed of cartilage. They pass at first backwards and then upwards
and inwards. They are homologous with part of the first branchial
arches of Selachians.

The columella and extra-columella have been already described (p.
251).

C. THE RIBS AND STERNUM.

=Thoracic ribs.=

The Crocodile has ten pairs of =thoracic ribs=, all except the last
one or two of which consist of three parts,--a vertebral rib, an
intermediate rib and a sternal rib.

Of the =vertebral ribs= the third may be taken as a type, it consists
of a curved bony rod which articulates proximally with the transverse
process of the vertebra by two facets. The terminal one of these, the
=capitulum= or =head=, articulates with a notch on the side of the
transverse process; the other, the =tuberculum=, which lies on the
dorsal surface a short distance behind the head, articulates with the
end of the transverse process. From near the distal end an imperfectly
ossified uncinate =process= (see p. 190) projects backwards.

The =intermediate ribs= are short and imperfectly ossified; they are
united with the =sternal ribs= (fig. 46, 3), which are large,
flattened, likewise imperfectly ossified structures, and articulate at
their distal ends with a pair of long divergent =xiphisternal horns=
(fig. 46, 5), which arise from the posterior end of the sternum
proper. The last pair of sternal ribs are attached to the preceding
pair, not to the xiphisternal horns.

The first and second vertebral ribs differ from the others in the fact
that the tuberculum forms a fairly long outstanding process.

=Cervical ribs.=

Movable ribs are attached to all the cervical as well as to the
thoracic vertebrae. Those borne by the atlas and axis are long, narrow
structures attached by a fairly broad base, and tapering gradually.
The ribs borne by the third to seventh cervical vertebrae are shaped
like a =T= with a double base, one limb of which, corresponding to the
tuberculum (fig. 41, 7), articulates with a short transverse process
arising from the neural arch, while the other, corresponding to the
capitulum, articulates with a surface on the centrum. The ribs
attached to the eighth and ninth cervical vertebrae are intermediate
in character between the =T=-shaped ribs and the ordinary thoracic ribs.
The anterior limb of the =T= is shortened, the posterior one is drawn
out, forming the shaft of the rib. The distal portion of the rib of
the ninth cervical vertebra is unossified.

The =Sacral ribs= have been described in connection with the sacral
vertebrae.

THE STERNUM.

The =sternum= of Crocodiles is a very simple structure, consisting of
a plate of cartilage (fig. 46, 2) lying immediately dorsal to the
interclavicle, and drawn out posteriorly into a pair of long
=xiphisternal horns= (fig. 46, 5).

THE ABDOMINAL SPLINT RIBS.

Lying superficially to the recti muscles of the ventral body-wall,
behind the sternal ribs, are seven or eight series of slender curved
bones, the _abdominal ribs_ (fig. 46, 4). Each series consists of four
or more bones, arranged in a =V=-like form with the angle of the =V=
directed forwards. They show a considerable amount of variability in
number and character. They are really membrane bones, and are in no
way homologous with true ribs, but correspond rather with the more
posterior of the bones constituting the plastron of Chelonia.

[Illustration FIG. 46. STERNUM AND ASSOCIATED MEMBRANE BONES OF A
CROCODILE (_C. palustris_) × 1/3. (Brit. Mus.)

The last pair of abdominal ribs which are united with the epipubes by
a plate of cartilage have been omitted.

    1. interclavicle.
    2. sternum.
    3. sternal rib.
    4. abdominal splint rib.
    5. xiphisternal horn.]

2. THE APPENDICULAR SKELETON.

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

[Illustration: FIG. 47. LEFT HALF OF THE PECTORAL GIRDLE OF AN
ALLIGATOR (_Caiman latirostris_) × 2/3. (Brit. Mus.)

    1. scapula.
    2. coracoid.
    3. interclavicle.
    4. glenoid cavity.]

THE PECTORAL GIRDLE.

The pectoral girdle of the Crocodile is less complete than is that of
most reptiles. It consists of a dorsal bone, the =scapula=, and a
ventral bone, the =coracoid=, with a median unpaired element, the
_interclavicle_; but there is no separate representative either of the
clavicle or precoracoid.

The =scapula= (fig. 47, 1) is a large bone, flattened and expanded
above where it is terminated by an unossified margin, the
=suprascapula=, and thickened below where it meets the coracoid. The
scapula forms about half the =glenoid cavity= (fig. 47, 4) for
articulation with the humerus, and has the lower part of its anterior
border drawn out into a roughened ridge.

The =coracoid= (fig. 47, 2) is a flattened bone, much expanded at
either end; it bears on its upper posterior border a flattened surface
which forms half the glenoid cavity, and is firmly united to the
scapula at its dorsal end. Its ventral end meets the sternum.

The _interclavicle_ (figs. 46, 1, and 47, 3) is a long narrow
blade-shaped bone lying along the ventral side of the sternum; about a
third of its length projects beyond the sternum in front.

THE ANTERIOR LIMB.

This is as usual divisible into three portions, the upper arm,
fore-arm and manus.

The =upper arm= or =brachium= contains one bone, the =humerus.=

The =humerus= (fig. 48, A, 1) is a fairly long stout bone,
considerably expanded at either end. The proximal end or head is
evenly rounded and is formed by an epiphysis ossifying from a centre
different from that forming the shaft. It articulates with the glenoid
cavity. The shaft bears on the flexor surface, at some little distance
behind the head, a prominent rounded protuberance, the =deltoid
ridge.= The distal end or trochlea is also formed by an epiphysis and
is partially divided by a groove into two convex surfaces; it
articulates with the two bones of the fore-arm, the radius and ulna.

[Illustration FIG. 48. A, RIGHT ANTERIOR, AND B, RIGHT POSTERIOR LIMB
OF A YOUNG ALLIGATOR (_Caiman latirostris_). (Brit. Mus.)

A × 1/2. B × about 1/3.]

    1. humerus.
    2. radius.
    3. ulna.
    4. radiale.
    5. ulnare.
    6. pisiform.
    7. patch of cartilage representing carpalia 1 and 2; between it
      and the radiale should be another flattened patch, the centrale.
    8. carpalia 3, 4, and 5 (fused).
    9. first metacarpal.
    10. proximal phalanx of second digit.
    11. second phalanx of fifth digit.
    12. femur.
    13. tibia.
    14. fibula.
    15. tibiale, intermedium and centrale (fused).
    16. fibulare.
    17. tarsalia 1, 2, and 3 (fused).
    18. tarsalia 4 and 5 (fused).
    19. first metatarsal.
    20. ungual phalanx of second digit.
    21. fifth metatarsal.

The =radius= and =ulna= are nearly equal in size and each consists of
a long shaft terminated at either end by an epiphysis.

The =radius= (fig. 48, A, 2) or pre-axial bone is slightly the smaller
of the two. It has a straight cylindrical shaft and is slightly and
nearly evenly expanded at either end. The proximal end which
articulates with the humerus is flat or slightly concave, the distal
end which articulates with the carpus is slightly convex.

The =ulna= (fig. 48, A, 3) or postaxial bone is a curved bone rather
larger than the radius. Its proximal end is large and convex, but is
not drawn out into an olecranon process.

The =Manus= consists of the =carpus= or =wrist=, and the =hand=.

The =Carpus=. This differs considerably from the more primitive type
met with in the Turtle. It consists of six elements arranged in a
proximal row of three and a distal row of two, with one intervening.
The bones of the proximal row are the radiale, the ulnare, and the
pisiform. The =radiale= (fig. 48, A, 4) is the largest bone of the
carpus: it is a somewhat hour-glass shaped bone, with its ends formed
by flattened epiphyses. It articulates by its proximal end with the
whole of the radius, and partly also with the ulna, and by its distal
end with the centrale.

The =ulnare= (fig. 48, A, 5) is a smaller bone, also somewhat
hour-glass shaped; it articulates proximally with the pisiform and
radiale, not quite reaching the ulna. The third bone of the proximal
row is the =pisiform= (fig. 48, A, 6), an irregular bone, articulating
with the ulna, radiale, and fifth metacarpal. The =centrale= is a
flattened cartilaginous element applied to the distal surface of the
radiale.

The distal row of carpals consists of two small structures. The first
of these forms a small cartilaginous patch, which is wedged in between
the first and second metacarpals, the centrale and the bone
representing carpalia 3, 4 and 5; this cartilaginous patch represents
=carpalia 1 and 2= (fig. 48, A, 7). The bone representing =carpalia 3,
4 and 5= is a good deal larger, rounded, and well-ossified; it
articulates with the ulnare, the pisiform, and the third, fourth, and
fifth metacarpals.

The =hand=. Each of the five digits consists of an elongated
metacarpal, terminated at each end by an epiphysis, and of a varying
number of phalanges. The terminal phalanx of each digit has an
epiphysis only at its proximal end, the others have them at both ends.

The first digit, or =pollex=, is the stoutest, and has two phalanges,
the second has three, the third four, the fourth three, and the fifth
two. The terminal phalanx of each of the first three digits is pointed
and sheathed in a horny claw; and is also marked by a pair of
prominent lateral grooves.

THE PELVIC GIRDLE.

The pelvic girdle of the Crocodile consists of four parts, a dorsal
element, the =ilium=, an anterior ventral element, the =pubis=, a
posterior ventral element, the =ischium=, and an accessory anterior
ventral element, the =epipubis=. All except the epipubis take part in
the formation of the =acetabulum=, which is perforated by a prominent
hole.

The =ilium= (fig. 49, 1) is a thick strong bone, firmly united on its
inner side with the two sacral ribs. Its dorsal border is rounded, its
ventral border bears posteriorly two irregular surfaces, completed by
epiphyses, which are united respectively with the ischium and pubis.

The =ischium= (fig. 49, 2)--the largest bone of the pelvis, is
somewhat contracted in the middle and expanded at either end. Its
proximal end, which is formed by an epiphysis, bears two surfaces, one
of which is united to the ilium, while the other forms part of the
acetabulum. The anterior border is also drawn out dorsally into a
strong process, which is terminated by a convex epiphysis, and is
united to the pubis. The ventral end of the ischium forms a flattened
blade, meeting its fellow in a median symphysis.

The =pubis= (fig. 49, 3) is much smaller than either the ilium or
ischium; it forms a small patch of unossified cartilage, interposed
between the anterior parts of the ilium and ischium.

[Illustration FIG. 49. PELVIS AND SACRUM OF AN ALLIGATOR (_Caiman
latirostris_) × 1/2. (Brit. Mus.)

    1. ilium.
    2. ischium.
    3. true pubis.
    4. epipubis (so-called pubis).
    5. acetabular foramen.
    6. neural spines of sacral vertebrae.
    7. symphysis ischii.
    8. process bearing prezygapophysis.]

The =epipubis= (fig. 49, 4) is a large bone with a thickened proximal
end, which is loosely articulated to the ischium, and a flattened
expanded distal end, which is united with its fellow, and with the
last pair of abdominal ribs by a large plate of cartilage. This bone
is generally described as the pubis.

THE POSTERIOR LIMB.

This is as usual divisible into three portions, the =thigh=, the
=crus= or =shin=, and the =pes=.

The =thigh= is formed by the =femur= (fig. 48, B, 12), a moderately
long stout bone, not unlike the humerus; it articulates with the
acetabulum by a fairly prominent rounded =head=. The distal end
articulating with the tibia and fibula is also expanded, and is
partially divided into equal parts by anterior and posterior grooves.
The flexor surface bears a fairly prominent trochanteric ridge. Each
end of the femur is formed by an epiphysis.

The =crus= or =shin= includes two bones, the =tibia= and =fibula=.
Both are well developed, but the tibia is considerably the larger of
the two.

The =tibia= (fig. 48, B, 13) is a strong bone with a flattened
expanded proximal end articulating with almost the whole of the end of
the femur, and a similarly expanded distal end articulating with a
bone representing the fused astragalus and centrale.

The =fibula= (fig. 48, B, 14) is flattened proximally, and articulates
with only quite a small part of the femur, while distally it is more
expanded, and articulates with the fibulare (calcaneum) and with a
facet on the side of the fused astragalus and centrale.

The =Pes= consists of the =tarsus= or =ankle=, and the =foot=.

The =Tarsus=. This, like the carpus, is much reduced and modified from
the primitive condition. It consists of only four bones, arranged in
two rows of two each. The two bones of the proximal row are much
larger than are those of the distal row. The pre-axial of them (fig.
48, B, 15) representing the fused =astragalus= (tibiale and
intermedium) and =centrale=, articulates proximally with the tibia and
fibula, and distally with the first metatarsal, and a small bone
representing the first three tarsalia. The postaxial bone, the
=calcaneum= (fibulare) (fig. 48, B, 16), is drawn out into a
prominent posterior process forming a heel such as is almost unknown
elsewhere except in mammals. It articulates with the fibula, the
tibiale-centrale, and distally with a bone representing the fourth and
fifth tarsalia, and with the fifth metatarsal.

The two bones forming the distal row of tarsals are both small and
rounded; one represents the first three tarsalia fused together, the
other tarsalia 4 and 5.

The =Foot=. The =foot= has five digits, but the fifth is much reduced,
consisting only of a short metatarsal. The first four =metatarsals=
are all long bones, slightly expanded at each end, and terminated by
small epiphyses. The first digit has two phalanges, the second three,
the third four, and the fourth five. The terminal or =ungual phalanx=
in each instance is grooved and pointed, and in the case of the first
three digits bears a horny claw. The ungual phalanx progressively
decreases in size from the first to the fourth. The fifth digit
consists only of a small, somewhat square metatarsal (fig. 48, B, 21),
attached to the bone representing the fused fourth and fifth
tarsalia.

FOOTNOTES:

[90] Free use has been made of L.C. Miall's _Studies in Comparative
Anatomy_, I., _The Skull in Crocodilia_, London, 1878. See also W.K.
Parker, _Tr. Z.S._, vol. XI. 1885, p. 263.

CHAPTER XVI.

GENERAL ACCOUNT OF THE SKELETON IN REPTILES.

EXOSKELETON.

The exoskeleton both epidermal and dermal is exceedingly well
developed in reptiles.

EPIDERMAL EXOSKELETON.

This generally has the form of overlapping horny =scales= which invest
outgrowths of the dermis, and are found covering the whole body in
most Rhynchocephalia, Ophidia, and Lacertilia, and many Crocodilia. In
the Ophidia the ventral surface of the tail is commonly covered by a
double row of broad scales, while the ventral surface of the precaudal
part of the body is covered by a single row. In the burrowing snakes
(Typhlopidae) and some sea snakes (Hydrophidae) these broad scales do
not occur, the scales of the ventral surface being similar to those of
the dorsal.

In the Chelonia with the exception of _Dermochelys_, _Trionyx_ and
their allies there is a well-developed system of horny shields having
a regular arrangement which has been described in the account of the
Turtle's skeleton[91].

The =rattle of the rattlesnake= is an epidermal structure formed of
several loosely articulated horny rings, produced by the modification
of the epidermal covering of the end of the tail, which instead of
being cast off when the rest of the outer skin is shed is retained
loosely interlocked with the adjoining ring or joint. New rings are
thus periodically added to the base of the rattle, and in old animals
the terminal ones wear away and are lost.

Horny claws occur on the ends of some or all of the digits in most
living reptiles.

Owen's Chameleon bears three epidermal horns, one arising from the
nasal and two from the frontal region.

In the Chelonia, some of the Theromorpha such as _Udenodon_ and
_Dicynodon_, probably also in the Pterosauria and _Polyonax_ among the
Dinosaurs, the jaws are more or less cased in horny beaks. The horny
beaks of Chelonia are variable; sometimes they have cutting edges,
sometimes they are denticulated, sometimes they are adapted for
crushing.

DERMAL EXOSKELETON.

Nearly all Crocodilia, many Dinosauria, some Rhynchocephalia and
Pythonomorpha, and some Lacertilia such as _Tiliqua_, _Scincus_ and
_Anguis_ have a dermal exoskeleton of bony scutes, developed below and
corresponding in shape to the epidermal scales. Sometimes as in
_Caiman sclerops_, _Jacare_ and _Teleosaurus_, the scutes completely
invest the body, being so arranged as to form a dorsal and a ventral
shield, and a continuous series of rings round the tail. In
_Crocodilus_ they are confined to the dorsal surface, and in
_Alligator_ to the dorsal and ventral surfaces. The scutes of some
extinct forms articulate with one another by a peg and socket
arrangement as in some Ganoid fish.

The =carapace= of most Chelonia is a compound structure, being partly
endoskeletal and formed from the ribs and vertebrae, partly from
plates derived from the dermal exoskeleton. The common arrangement is
seen in fig. 36. All the surface plates are probably exoskeletal in
origin, but united with the ventral surfaces of the costal and neural
plates respectively are the expanded ribs and neural arches of the
vertebrae.

The =plastron= in the common genus _Chelone_ (fig. 37) includes nine
plates of bone, one unpaired and four pairs; they will be referred to
in connection with the ribs and pectoral girdle.

In the Leathery Turtle (_Dermochelys_) the carapace and plastron
differ completely from those of any other living form. The carapace
consists of a number of polygonal ossifications fitting closely
together and altogether distinct from the vertebrae and ribs. The
plastron is imperfectly ossified, and not united with the pelvis, and
the whole surface of both carapace and plastron is covered with a
tough leathery skin, without horny shields.

Some of the extinct Dinosauria have an enormously developed dermal
exoskeleton. Thus in _Stegosaurus_ and _Omosaurus_ the dorsal surface
is provided with flattened plates or with spines reaching a length of
upwards of two feet. In _Polacanthus_ the posterior part of the body
is protected by a bony shield somewhat recalling that of the little
armadillo _Chlamydophorus_. No exoskeleton is known in Ichthyosauria,
Sauropterygia, Pterosauria, many Dinosauria and Theromorpha, and some
Lacertilia, such as _Chamaeleon_ and _Amphisbaena_.

TEETH.

The teeth of reptiles are generally well developed, and in the great
majority of forms are simple conical structures, uniform in character,
generally somewhat recurved, and often with serrated edges. Another
common type of tooth is that with a laterally compressed triangular
crown provided with a double cutting edge which may or may not be
serrated. The teeth are mainly formed of dentine, with usually an
external layer of enamel, and often a coating of cement on the root.
Vasodentine is found below the dentine in _Iguanodon_. The teeth of
reptiles never have the enamel deeply infolded, nor do they have
double roots.

Teeth may be present not only on the jaw-bones, but as in many
_Squamata_, also on the palatines, pterygoids or vomers. The method
by which they are attached to the bones varies much. Sometimes as in
_Iguana_ and some other lizards, they are pleurodont[1], sometimes
they are acrodont[92], as in the Rhynchocephalia, Pythonomorpha,
Ophidia and some Lacertilia such as _Agama_. Again they may be set in
a continuous groove as in the Ichthyosauria and young Crocodilia.
Finally the teeth may be _thecodont_ or placed in distinct sockets as
in the Theromorpha, Sauropterygia, adult Crocodilia, Sauropoda and
Theropoda. In _Iguanodon_ the teeth are set in shallow sockets in a
groove one side of which is higher than the other; the method of
attachment thus shows points of resemblance to the thecodont
condition, the pleurodont condition, and that met with in the
Ichthyosauria.

In _Ichthyosaurus_ the teeth are marked by a number of vertical
furrows, and it is from a furrow of this nature greatly enlarged and
converted into a tube that the channel down which flows the poison of
venomous snakes is derived.

In most reptiles the dentition is more or less homodont. The only
reptiles in which a definite heterodont dentition is known are the
extinct Theromorpha, and in them the teeth vary greatly. Thus
_Udenodon_ is toothless, the jaws having been probably cased in a
horny beak. In _Dicynodon_ the jaws are likewise toothless with the
exception of a pair of permanently growing tusks borne by the maxillae.
_Dicynodon_ is the only known reptile whose teeth have permanently
growing pulps. In _Pariasaurus_ the teeth are uniform and very
numerous, and though placed in distinct sockets are ankylosed to the
jaw. In _Galesaurus_ and _Cynognathus_ three kinds of teeth can be
distinguished, slender conical incisor-like teeth, large canine-like
teeth, and cheek teeth with two or three cusps. The teeth in
_Galesaurus_ are confined to the jaws, in _Placodus_ and its allies,
however, large flat crushing teeth are attached to the palatines as
well as to the jaw-bones, and in _Pariasaurus_ the vomer, palatine and
pterygoid all bear teeth as well as the jaw bones. The upper jaw of
_Sphenodon_ and other Rhynchocephalia is provided with two parallel
rows of teeth, one borne on the maxillae and one on the palatines, the
mandibular teeth bite in a groove between these two rows. The bone of
the jaws in _Sphenodon_ is so hard that when the teeth get worn away,
it can act as a substitute. In the young _Sphenodon_ the vomers bear
teeth, as they do also in _Proterosaurus_.

There is generally a continuous succession of teeth throughout life,
the new tooth coming up below, or partly at the side of the one in
use, and causing the absorption of part of its wall or base. In this
way the new tooth comes to lie in the pulp cavity of the old one. This
method of succession is well seen in the Crocodilia.

[Illustration FIG. 50. PREPARATION OF PART OF THE RIGHT MANDIBULAR
RAMUS OF _Crocodilus palustris_ × 1/2. (Brit. Mus.)

    1. tooth in use.
    2. fairly old germ of future
    tooth.
    3. symphysial surface of the
    mandible.]

Teeth have been detected in embryos of _Trionyx_, but otherwise no
teeth are known to occur in Chelonia, or in _Pteranodon_
(Pterosauria), while the anterior part of the jaw is edentulous in
_Iguanodon_, _Polyonax_ and some other Dinosaurs, and in
_Rhamphorhynchus_.

ENDOSKELETON.

VERTEBRAL COLUMN.

The vertebral column is commonly divisible into the usual five
regions, but in the Ophidia, Ichthyosauria, and Amphisbaenidae among
Lacertilia, only into caudal and precaudal regions. In the Chelonia
there are no lumbar vertebrae.

The form of the vertebral centra is very variable. A large proportion
of extinct reptiles,--several entire orders,--and the earlier and more
primitive forms in some of the other groups have amphicoelous
vertebrae. Vertebrae of this type occur in the Theromorpha,
Ichthyosauria, most Sauropterygia and Rhynchocephalia, and many
Dinosauria, also in some of the early Crocodilia such as _Belodon_,
_Teleosaurus_ and _Goniopholis_, and the Geckonidae among Lacertilia.

The majority of living reptiles have procoelous vertebrae. Thus they
occur in the Lacertilia (excluding the Geckos), the Ophidia, and the
Crocodilia, also among extinct forms in the Pterosauria and many
Dinosauria. On the other hand some Dinosauria such as _Iguanodon_ have
opisthocoelous cervical vertebrae, while others have opisthocoelous
thoracic vertebrae. The vertebrae of the Ceratopsidae and some
Sauropterygia, the thoracic vertebrae of _Iguanodon_, and the sacral
vertebrae of Crocodilia have flat centra. The first caudal vertebra of
modern Crocodilia is biconvex, and in the Chelonia all types of
vertebral centra are found. The cervical vertebrae of _Sphenodon_ are
noticeable for the occurrence of a small pro-atlas, which may
represent the neural arch of a vertebra in front of the atlas.

In most reptiles the vertebrae are fully ossified, but in some of the
more primitive forms the notochord persists in the centre of the
vertebra (i.e. intervertebrally), this is the case for instance in
many of the Theromorpha and Rhynchocephalia, and also in the Geckos.
In other reptiles it persists longest intravertebrally.

The centrum of each of the caudal vertebrae of most Lacertilia is
traversed by an unossified septum along which it readily breaks.

Chevron bones occur below the caudal vertebrae in Lacertilia,
Chelonia, Ichthyosauria, many Dinosauria, and _Sphenodon_,
articulating with quite the posterior part of the centrum which bears
them. In Lacertilia and Crocodilia (fig. 41, 3) the axis has a
well-marked odontoid process. The ventral portions of the
intervertebral discs are sometimes ossified, forming wedge-shaped
inter centra, as in Geckos, and the cervical vertebrae of _Sphenodon_.

In snakes, Theropod Dinosaurs, and the iguanas among lizards, the
neural arches are provided with _zygosphenes_, and _zygantra_.

The neural arches are usually firmly ankylosed to the centra, but in
the Crocodilia and some Chelonia, Sauropterygia, and Dinosauria, the
suture between the centrum and neural arch persists at any rate till
late in life. In the Ichthyosauria the neural arches were united to
the centra by cartilage only.

The thoracic vertebrae of some of the Theromorpha (_Dimetrodon_) are
remarkable for the extraordinary development of the neural spine, and
those of Chelonia for the absence of transverse processes.

In living reptiles the number of sacral vertebrae is nearly always
two, but in the Theromorpha, Dinosauria, and Pterosauria, as many as
five or six bones may be ankylosed together in the sacral region. In
Crocodiles the two halves of the pelvis sometimes articulate with
different vertebrae. The vertebrae of some of the great Sauropoda are
remarkably hollowed out, having a large vacuity on each side of the
centrum communicating with a series of internal cavities. The whole
structure of these vertebrae shows a combination of great strength
with lightness.

THE SKULL.

The reptilian skull is well ossified and the bones are noticeable for
their density. The true cranium is often largely concealed by a
secondary or false roof of membrane bones, which is best seen in the
Ichthyosauria and some of the Chelonia. In other reptiles the false
roof is more or less broken up by vacuities exposing the true cranial
walls. The ethmoidal region is the only one in which much of the
primordial cartilaginous cranium remains. The lateral parts of the
sphenoidal region are also as a rule not well ossified.

In some reptiles, such as most Lacertilia and Chelonia, the orbits are
separated only by the imperfect interorbital septum, while in others,
such as the Ophidia, Crocodilia and Amphisbaenidae, the cranial cavity
extends forwards between the orbits.

In the occipital region all four bones are ossified. The great
majority of reptiles have a single convex occipital condyle, but some
of the Theromorpha such as _Cynognathus_ have two distinct condyles as
in mammals. Sometimes, as in Chelonia, Ophidia and Lacertilia, the
exoccipitals, as well as the basi-occipital, take part in the
formation of the single condyle; sometimes, as in Crocodiles, it is
formed by the basi-occipital alone, as in birds. The relations of the
bones to the foramen magnum vary considerably, in Chelonia the
basi-occipital generally takes no part in bounding it, and in the
Theromorpha, Crocodilia, and Ophidia, the supra-occipital is excluded.
The parietals are paired in Geckos and Chelonia alone among living
forms, and in the extinct Ichthyosauria and some Theromorpha; in all
other reptiles they are united.

The frontals are paired in Ichthyosauria (fig. 32, 5), Chelonia,
Ophidia, _Sphenodon_ (fig. 52, B, 4) and some extinct crocodiles, such
as _Belodon_. They are completely fused in living Crocodilia and some
Lacertilia and Dinosauria. In the gigantic _Polyonax_ they are drawn
out into a pair of enormous horns, and the parietals and squamosals
are greatly expanded behind.

An interparietal foramen occurs in the Theromorpha, the Ichthyosauria
(fig. 32, 10), _Sphenodon_, the Sauropterygia and most Lacertilia.
The posterior part of the skull is curiously modified in some
Chamaeleons, the parietals and supra-occipitals being drawn out into a
backwardly-projecting sagittal crest which unites with the two
prolongations from the squamosals. In other Chamaeleons (_C. bifidus_)
prolongations of the prefrontals and maxillae form large
forwardly-projecting bony processes.

The roof of the skull is characterised by the development of
prefrontals and postfrontals, which lie respectively near the anterior
and posterior extremity of the orbit. In Theromorpha, Squamata,
Crocodilia, and some Dinosauria lachrymals are developed. There is a
ring of bones in the sclerotic in the Ichthyosauria (fig. 32, 15), the
Metriorhynchidae among Crocodiles and some Rhynchocephalia,
Dinosauria, and Pterosauria.

The pro-otic lies in front of the exoccipital and together with the
opisthotic forms the hind border of the fenestra ovalis. In Chelonia
the opisthotic remains separate, in all other living reptiles it fuses
with the exoccipital. The epi-otic fuses with the supra-occipital.

The parasphenoid, so important in Ichthyopsids, has very often
disappeared completely; it is present, however, in the Ichthyosauria,
the Plesiosauridae, and a number of Squamata, in many Ophidia its
anterior part forming the base of the interorbital septum.

In the Plesiosauridae and most Lacertilia, but not in the
Amphisbaenidae, a slender bone, the epipterygoid, occurs uniting the
parietal or the anterior end of the pro-otic with the pterygoid. A
homologous arrangement occurs in the Ichthyosauria and some Chelonia.

In most reptiles a transpalatine occurs, connecting the maxillae with
the pterygoid, but this is absent in the Chelonia, and some
Dinosauria, and in the Typhlopidae among snakes.

The quadrate is always well developed, and except in the Squamata is
firmly fixed to the surrounding bones. The Chamaeleons also, among the
Squamata, have a fixed quadrate, and in them too the quadratojugal is
absent. Separate nasal bones do not occur in any living Chelonia.

The vomers are generally paired as in Squamata, sometimes unpaired as
in Chelonia.

[Illustration FIG. 51. DORSAL (TO THE LEFT) AND VENTRAL (TO THE RIGHT)
VIEWS OF THE SKULL OF THE COMMON SNAKE (_Tropidinotus natrix_). (After
PARKER.)

     1. premaxillae (fused).
     2. anterior nares.
     3. nasal.
     4. prefrontal.
     5. frontal.
     6. parietal.
     7. maxillae.
     8. transpalatine.
     9. palatine.
    10. pterygoid.
    11. pro-otic.
    12. exoccipital.
    13. supra-occipital.
    14. opisthotic.
    15. epi-otic.
    16. quadrate.
    17. parasphenoid.
    18. basisphenoid.
    19. basi-occipital.
    20. occipital condyle.
    21. splenial.
    22. dentary.
    23. angular.
    24. articular.
    25. supra-angular.
    26. coronoid.
    27. vomer.
    28. squamosal.

    IX, X foramina for the ninth
    and tenth cranial nerves.]

The disposition of the bones of the jaws is subject to much
modification in the Ophidia in order to adapt them for swallowing very
large prey. The arrangements again differ greatly in the venomous and
non-venomous snakes. In the non-venomous snakes, such as _Python_ and
_Tropidonotus_, the palatine is large and is fixed to the pterygoid
which extends outwards (fig. 51, 10) so as to be united to the
quadrate, and is at the same time firmly connected by the
transpalatine with the maxillae. The quadrate is united to the
squamosal, which is loosely attached to the cranium. The premaxillae is
moderately developed and bears teeth, and the maxillae forms a long bar
loosely connected with the rest of the skull. The rami of the mandible
are united only by an extremely elastic ligament. It is as regards the
maxillae and premaxillae that the skulls of venomous and non-venomous
snakes differ most. In the rattlesnake (_Crotalus_) and other venomous
snakes the premaxillae is extremely small and toothless. The maxillae is
small and subcylindrical, and is movably articulated to the lachrymal,
which also is capable of a certain amount of motion on the frontal.
The maxillae is connected by means of the transpalatine with the
pterygoid, which in its turn is united to the quadrate. When the mouth
is shut the quadrate is directed backwards, and carrying back the
pterygoid and transpalatine pulls at the maxillae and causes its
palatal face, to which the poison teeth are attached, to lie back
along the roof of the mouth. When the mouth opens the distal end of
the quadrate is thrust forward, and this necessitates the pushing
forward of the pterygoid and transpalatine, causing the tooth-bearing
surface of the maxillae to look downwards and the tooth to come into
the position for striking.

The Ophidian skull is also noticeable for the absence of the jugals
and quadratojugals. In poisonous snakes the place of the jugal is
taken by the zygomatic ligament which connects the quadrate and
maxillae.

The extent to which the palate is closed in reptiles varies much. In
many reptiles, such as the Squamata and Ichthyosauria, the palate is
not complete, both palatines and pterygoids being widely separated in
the middle line. In others, such as the Crocodilia, Sauropterygia, and
Chelonia, there is a more or less complete bony palate. In many
Chelonia this is chiefly formed of the vomer, palatines, and
pterygoids, the posterior nares being mainly bounded by the palatines.
In living Crocodilia, however, outgrowths are formed from the
pterygoids and palatines which arch round and meet one another
ventrally, forming a secondary palate (fig. 43, A), which completely
shuts off the true sphenoidal floor of the skull, and causes the
posterior nares which are bounded by the pterygoids to open very far
back. Though this feature is common to all postsecondary crocodiles,
it is interesting to notice that it is not found in the earlier forms,
but that its gradual evolution can be traced. In the Triassic
_Belodon_, for instance, the posterior nares open far forwards, and
are not surrounded by either the palatines or pterygoids. In the
Jurassic crocodile, _Teleosaurus_, the posterior nares lie further
back, being surrounded by the palatines, but the pterygoids do not
meet them. Finally, in the Tertiary forms the arrangements are as in
living crocodiles.

A short secondary hard palate is found also in the Theriodontia. The
palatines of _Ichthyosaurus_ are noticeable for their transverse
position, which recalls that in the Frog.

The various =fossae= or =vacuities= in the false roof of the skull are
important, and their relations may best be understood by a description
of their mode of occurrence in _Sphenodon_, a form in which they are
very completely developed.

[Illustration FIG. 52. SKULL OF HATTERIA. (_Sphenodon punctatus_). A,
lateral; B, dorsal; C, ventral; D, posterior. (After VON ZITTEL.)

     1. premaxillae.
     2. nasal.
     3. prefrontal.
     4. frontal.
     5. postfrontal.
     6. parietal.
     7. squamosal.
     8. quadratojugal.
     9. quadrate.
    10. postorbital.
    11. jugal.
    12. maxillae.
    13. vomer.
    14. palatine.
    15. pterygoid.
    16. transpalatine.
    17. exoccipital.
    18. epipterygoid.
    19. basisphenoid.
    20. supratemporal fossa.
    21. infratemporal or lateral temporal
    fossa.
    22. orbit.
    23. post-temporal fossa.
    24. foramen magnum.
    25. anterior nares.
    26. interparietal foramen.
    27. dentary.
    28. supra-angular.
    29. articular.]

In _Sphenodon_, then, on the dorsal surface of the skull, are the
large =supratemporal fossae= (fig. 52, 20). Their inner margins are
separated from one another by the parietal walls of the cranium, while
externally each is bounded by a bony arch, the =supratemporal arcade=,
formed of the postfrontal, postorbital, and squamosal. Posteriorly the
boundary is formed by a =post-temporal bar=, formed by the parietal
and squamosal. Below the supratemporal arcade is another large
vacuity, the =infratemporal= or =lateral temporal fossa= (fig. 52,
21). This is bounded above by the supratemporal arcade, and is
separated from the orbit in front by the =postorbital bar=, formed by
the union of outgrowths from the jugals and postorbitals. Behind it is
bounded by a continuation of the post-temporal bar formed of the
squamosal and quadratojugal, and below by an =infratemporal arcade=,
which is chiefly composed of the quadratojugal and jugal.

Below the post-temporal bar is a third vacuity, the =post-temporal
fossa= (fig. 52, D, 23), bounded above by the post-temporal bar and
below by the exoccipital and opisthotic.

_Sphenodon_ and the Crocodilia are the only living reptiles with
complete supratemporal and infratemporal arcades, but they are both
present in the extinct Pterosauria and some Dinosauria.

Supratemporal fossae, bounded below by supratemporal arcades, occur in
all reptiles except some Chelonia, the Ophidia, the Geckonidae among
Lacertilia, and the Pariasauria and others among Theromorpha; they are
specially large in _Nothosaurus_ among the Sauropterygia, _Dicynodon_
among the Theromorpha, and many Crocodilia and Pterosauria. In some
Dinosaurs, such as _Ceratosaurus_, they are very small, while the
infratemporal fossae are correspondingly large.

In _Elginia_[93] (Theromorpha) and some Chelonia, such as _Chelone_,
there are no fossae on the surface of the skull, a complete false roof
being developed; in other Chelonia, such as _Trionyx_, the true
cranium is freely visible, the only part of the false roof developed
being the infratemporal arcade.

In many reptiles large =pre-orbital vacuities= occur; they are
specially large in the Pterosauria and in some of the Crocodilia and
Dinosauria (fig. 35, 3). In some Pterosauria they are confluent with
the orbits.

The premaxillae are usually separate, but sometimes, as in some Ophidia
(fig. 51, 1), Chelonia, Lacertilia (Agamidae), and Dinosaurs
(Ceratopsia) they are united. In the Dinosaur _Hadrosaurus_ they are
exceedingly large and spatulate. In the Rhynchocephalian
_Hyperodapedon_ they are drawn out into a strongly curved beak.

As regards the mandible, sometimes, as in most Rhynchocephalia,
Ophidia and Pythonomorpha, the rami have only a ligamental union;
sometimes, as in Crocodilia, the Rhynchosauridae and the majority of
Lacertilia, they are suturally united. In Chelonia (fig. 28, B, 12),
and apparently in Pterosauria, the two dentaries are completely fused
together. The sutures between the various bones of the lower jaw
usually persist, but in Ophidia those between the angular,
supra-angular, articular and coronoid are obliterated. There are
sometimes large vacuities in the mandible, as in Theromorpha,
Crocodilia, and some Dinosauria. In _Iguanodon_, _Polyonax_,
_Hypsilophodon_ and _Hadrosaurus_ among Dinosaurs the mandible has a
predentary or mento-meckelian bone which, in some cases at any rate,
was probably sheathed in a horny beak.

The principal part of the auditory ossicular chain is formed by a
rod-like columella. The development of the hyoid apparatus varies, and
it often happens that the first branchial arch is better developed
than is the hyoid arch. In the Crocodilia and Chelonia there is a
large basilingual plate or body of the hyoid (fig. 53, 1); but while
in the Crocodilia the first branchial forms the only well-developed
arch, in the Chelonia the first and second branchials are both
strongly developed, and the hyoid is often fairly large.

THE RIBS.

[Illustration FIG. 53. HYOIDS OF AN ALLIGATOR (_Caiman latirostris_)
(TO THE LEFT) AND OF A GREEN TURTLE (_Chelone midas_) (TO THE RIGHT) ×
5/8. (Brit. Mus.)

The cartilaginous portions are dotted.

    1. basilingual plate or body of
    the hyoid.
    2. hyoid arch.
    3. first branchial arch (anterior
    cornu).
    4. second branchial arch (posterior
    cornu).]

Ribs are always present, and may be attached to any of the precaudal
vertebrae. In most reptiles the posterior cervical vertebrae bear
ribs, while the atlas and axis are ribless; in Crocodiles and Geckos,
however, ribs are borne even by the atlas and axis. On the other hand,
in the Chelonia none of the cervical vertebrae bear obvious ribs. In
the following groups the thoracic ribs have both capitula and
tubercula--Theromorpha, Ichthyosauria, Crocodilia, Dinosauria,
Pterosauria. In the other groups each rib articulates by a single
head, and the position of the facet is subject to a considerable
amount of variation, thus in the Squamata it lies on the centrum, and
in the Sauropterygia on the neural arch, while in the Chelonia the rib
articulates with the contiguous parts of two centra instead of
directly with one.

In most reptiles a greater or smaller number of ribs are united
ventrally with a sternum; but in snakes a continuous series of similar
ribs, all articulating freely with the vertebral column, extends from
the third cervical vertebra to the end of the trunk. The number of
ribs connected with the sternum varies from three or four in Lizards
to eight or nine in Crocodiles. Those which reach the sternum are
nearly always divided into vertebral, sternal, and intermediate
portions, and as a rule only the vertebral portion is completely
ossified. In Crocodiles a number of sternal ribs are connected with a
cartilaginous arch, which is attached to the hind end of the sternum,
and represents the xiphisternum. The sacral ribs connecting the
vertebral column with the ilia are very distinct in Crocodiles; in
these animals and _Sphenodon_ the vertebral ribs have backwardly
projecting uncinate processes as in birds.

In the curious arboreal lizard, _Draco volans_, the posterior ribs are
long and straight, and support a parachute-like expansion of the
integument used in its long flight-like leaps. In Chelonia the ribs
are generally combined with the carapace.

In Ichthyosauria, Sauropterygia, Crocodilia and _Sphenodon_, abdominal
splint ribs occur; and probably all except the first of the paired
ossifications forming the plastron of Chelonia are of similar
character. Abdominal ribs have quite a different origin from true
ribs, for while true ribs are cartilage bones, abdominal ribs have no
cartilaginous precursors, but are simply the ossified tendons of the
rectus abdominalis muscle.

THE STERNUM.

A sternum occurs in the following groups of reptiles: Rhynchocephalia,
nearly all Lacertilia, Pythonomorpha, Crocodilia, and Pterosauria, and
is generally more or less rhomboidal or shield-shaped. In Pterosauria
it is keeled and bears some resemblance to that of birds. It may have
been replaced by membrane bone.

[Illustration FIG. 54. VENTRAL VIEW OF THE SHOULDER-GIRDLE OF STERNUM
OF A LIZARD (_Loemanctus longipes_) × 2. (After PARKER.)

    1. interclavicle.
    2. clavicle.
    3. scapula.
    4. coracoid.
    5. precoracoidal process.
    6. glenoid cavity.
    7. sternum.
    8. xiphisternum.
    9. sternal rib.]

The sternum is absent in Sauropterygia, Ichthyosauria, Chelonia,
Ophidia, and most of the snake-like Amphisbaenidae among Lacertilia;
while it is not well known in Theromorpha and Dinosauria. In the
Sauropod _Brontosaurus_, however, two rounded bones occur near the
base of the coracoids, and these probably represent ossified patches
in a sternum, which was mainly cartilaginous; similar structures occur
in _Iguanodon_.

The sternum frequently remains wholly cartilaginous, especially in
Lacertilia; sometimes it becomes calcified, but true ossification does
not as a rule take place.

APPENDICULAR SKELETON.

THE PECTORAL GIRDLE.

The pectoral girdle is well developed in all groups of reptiles except
the Ophidia, occurring even in the limbless Amphisbaenidae. It is very
solid in the Theromorpha. As a rule all three cartilage bones,
scapula, coracoid, and precoracoid are represented, and frequently
also the membrane bones,--clavicles, and interclavicle.

The coracoids are generally flat expanded bones, which sometimes, as
in Sauropterygia and Ichthyosauria, meet in a ventral symphysis;
sometimes, as in Lacertilia, are united with the sides of the sternum.
In Chelonia neither the coracoids nor precoracoids meet one another,
but their free ends are connected by fibrocartilaginous bands. In
Lacertilia the coracoids are pierced by fenestrae.

The precoracoid is generally represented, but the Theromorpha are the
only reptiles in which it is separately ossified; it forms a
well-marked process on the coracoid in Lacertilia (fig. 54, 5). It is
absent in Ichthyosauria, and Dinosauria, and probably in
Sauropterygia. In some Lacertilia and Chelonia the sternal ends of the
coracoids are unossified and form epicoracoids; in some Chelonia there
are also epiprecoracoids; but neither these nor the epicoracoids
overlap their fellows of the opposite side as they do in arciferous
Anura (see p. 185). In some Lacertilia with degenerate limbs the
pectoral girdle is also much reduced, in _Ophisaurus apus_ the ventral
borders of the coracoids are widely separated.

A scapula is always present, and is generally expanded distally, but
in the Chelonia the distal end is cylindrical. In the Theromorpha it
has an acromial process with which the precoracoid articulates, and it
is very large in Dinosauria. In the Chelonia the scapula and
precoracoid are ossified continuously. Among the Pterosauria,
_Pteranodon_ has an unique pectoral girdle; the scapula and coracoid
are ankylosed and the scapula articulates with the neural spines of
several ankylosed vertebrae.

Clavicles occur in some Theromorpha such as _Pariasaurus_, and also in
the Ichthyosauria, Sauropterygia, Rhynchocephalia, and most
Lacertilia. They are absent in the Pterosauria, the Chamaeleons among
Lacertilia, the Ophidia and the Crocodilia. They are wanting too in
the Chelonia, unless the first pair of ossifications in the plastron
are to be regarded as clavicles. In the Sauropterygia bones regarded
as the clavicles and interclavicle are generally well developed. The
unpaired ossification in the plastron of Chelonia is an interclavicle,
and a representative of the same bone occurs arising from the sternum
in Pterosauria. A well developed =T=-shaped interclavicle is found in
Ichthyosauria, Rhynchocephalia, Lacertilia, and some Theromorpha, such
as _Pariasaurus_.

THE LIMBS.

In most reptiles there are two pairs of pentedactylate limbs provided
with claws, but in nearly all Ophidia and some Lacertilia
(_Amphisbaena_, _Lialis_, _Anguis_) the limbs have entirely
disappeared. In a few Ophidia such as _Python_ traces of the posterior
limbs occur, and in _Chirotes_ among the Amphisbaenidae there are
minute anterior limbs. The Lacertilians, _Chalcides_ (_Seps_) and
_Ophisaurus_ (_Bipes_, _Pseudopus_) have very small posterior limbs.

The limbs are as a rule adapted for walking, but in Ichthyosauria,
Sauropterygia, Pythonomorpha and some Chelonia, they have the form of
swimming paddles, the relative size of the manus and pes being
increased, while that of the proximal and middle portions of the limbs
is reduced. This reduction is carried to its furthest extent in the
Ichthyosauria in which radius and ulna, tibia and fibula, have the
form of short polygonal bones similar to those constituting the manus
and pes. In the Pythonomorpha the reduction of the limb bones is not
quite so marked, in the Sauropterygia it is less, and still less in
the Chelonia. In the earlier Ichthyosauria too, the limb bones are not
so short as they are in the later forms. The Ichthyosaurian limb is
also remarkable, firstly for the fact that both humerus and femur are
terminated by concave articulating surfaces instead of by convex
condyles, and secondly for the great multiplication of the phalangeal
bones, each digit being sometimes composed of a series of over twenty.
Sometimes too the number of series is increased, either by the
bifurcation of some of the digits or by the development of marginal
bones. In the Sauropterygia the phalanges are likewise increased above
the normal but not so much as in Ichthyosauria. The humerus and femur
of Sauropterygia are noticeable for the enormous size of the terminal
epiphyses which form in each case by far the greater part of the bone.

THE ANTERIOR LIMB.

The anterior limb is usually approximately equal in length to the
posterior, but in many Dinosauria it is considerably the shorter of
the two. The humerus is generally without distinct condyles, but they
are well developed in the Theromorpha, the Lacertilia and _Sphenodon_.

In the Theromorpha, some Rhynchocephalia, and some Sauropterygia, such
as _Mesosaurus_, the humerus has an ent-epicondylar foramen; in
Lacertilia, Chelonia and some Dinosauria there is an ect-epicondylar
foramen or groove; _Sphenodon_ possesses both ent- and ect-epicondylar
foramina. The radius and ulna are always separate. In some Chelonia,
such as _Chelydra_, the carpus has a very simple arrangement, namely,
a proximal row of three bones, the radiale, intermedium and ulnare,
and a distal row of five carpalia, with one bone, the centrale,
between the two rows. Many reptiles have a carpus only slightly
different from this. Thus the carpus in _Sphenodon_ differs mainly in
having two centralia, that of most Lacertilia, in having the centrale
and intermedium fused.

Crocodiles have a much reduced carpus with the radiale and ulnare
considerably elongated. The manus in Chamaeleons is curiously
modified, having the first three digits arranged in one group and
turned inwards, and the fourth and fifth in another group turned
outwards; carpalia 3 and 4 are united.

In the Pterosauria the anterior limbs form wings, the phalanges of the
fifth digit being very greatly elongated to support the wing membrane.
The first digit is vestigial and the second, third, and fourth are
clawed.

THE PELVIC GIRDLE.

The pelvic girdle is well developed in all reptiles which have
posterior limbs, but is absent or quite vestigial in Ophidia and those
Lacertilia which have no posterior limbs. The ilium and ischium agree
in their general characters throughout all the various groups of
reptiles, but that is not the case with the pubis.

In many reptiles such as Chelonia, Ichthyosauria and Lacertilia the
ilia are small, more or less cylindrical bones either directed
backwards, or vertically placed as in the Chamaeleons. In the
Crocodilia they are larger and more expanded, while in Dinosauria and
Pterosauria they are greatly elongated both in front of, and behind,
the acetabulum. The ischia are generally strongly developed somewhat
square bones meeting in a ventral symphysis. In Dinosauria the ischium
(fig. 35, 9) is a much elongated and backwardly-directed bone,
bearing a forwardly projecting obturator process. In Pterosauria the
ischium is fused with the ilium, and in both pterosaurs and crocodiles
the ilium and ischium are the only bones taking part in the formation
of the acetabulum. In most Lacertilia there is an unpaired structure,
the _hypo-ischium_ or _os cloacae_ projecting back from the symphysis
ischii, which is usually separated from the symphysis pubis by a large
space, the _foramen cordiforme_. In some Lacertilia and Chelonia there
is a cartilaginous bar dividing the foramen cordiforme into two
obturator foramina; in many Chelonia this bar is ossified. Among
_Ophidia_, _Python_, _Tortrix_, _Typhlops_ and their allies have a
structure representing a vestigial ischio-pubis: but in most Ophidia
there is no trace of the pelvis. In some Theromorpha all the bones of
the pelvis are completely fused, forming an os innominatum as in
mammals; the pubes and ischia are so completely fused that sometimes
as in _Pariasaurus_ even the obturator foramina are closed.

Concerning the reptilian pubis there are considerable difficulties.
Sometimes there is only a single pubic structure present, sometimes
there are two. The reptilian pubis is best understood by comparing the
arrangements met with in the various other groups with that in the
Orthopod Dinosaurs such as _Iguanodon_. In _Iguanodon_ the pubis
consists of two portions, viz. of a moderately broad pre-pubis
directed downwards and forwards, and of a narrow greatly elongated
post-pubis directed backwards parallel to the ischium. The pubis is
united to both ilium and ischium, the acetabulum has a large
unossified space, and neither pre-pubes nor post-pubes meet in ventral
symphyses. The arrangement bears a great resemblance to that of Ratite
birds. In Lacertilia, Chelonia, Rhynchocephalia and Ichthyosauria
together with Theropod and Sauropod Dinosaurs the pubis corresponds to
the pre-pubis of _Iguanodon_ and is a more or less cylindrical bone
expanded at both ends, meeting its fellow in a ventral symphysis. In
Chelonia and Lacertilia the pubis bears a lateral process which is
homologous with the post-pubis of Iguanodon. In Lacertilia and
sometimes in Chelonia there is a cartilaginous epipubis attached to
the anterior border of the pubic symphysis; this is well developed in
the Chamaeleons and Geckos. In Crocodilia there is, forming the
anterior and ventral portion of the acetabulum, a patch of cartilage
(fig. 49, 3) which is probably the true pubis homologous with that of
lizards and with the pre-pubis of _Iguanodon_. The large bone
generally called the pubis in Crocodiles is probably an epipubis.

THE POSTERIOR LIMB.

The posterior limb is entirely absent in some Lacertilia and in most
Ophidia, though traces occur in _Python_, _Tortrix_ and _Typhlops_. In
the Ichthyosauria, Sauropterygia and Pythonomorpha the posterior limbs
form swimming paddles and have been already referred to.

The arrangement of the proximal and middle segments of the limb is
fairly constant in all reptiles with limbs adapted for walking, and
the tibia and fibula are always separate. The pes is however subject
to a considerable amount of variation, especially as regards the
tarsus. In some Chelonia the tarsus like the carpus has an extremely
simple arrangement, consisting of a proximal row of three bones, the
tibiale, intermedium and fibulare, a centrale, and a distal row of
five tarsalia. In most living reptiles, however, the tibiale and
intermedium are as in mammals united, forming the astragalus. In
Crocodiles (fig. 48, B, 15) the centrale is also united with the
tibiale while the distal tarsalia are very slightly developed. The
calcaneum in Crocodiles is drawn out into a long process forming a
heel in a manner almost unique among Sauropsida. In _Sphenodon_ and
Lacertilia the tibia and fibula articulate with a single large bone
representing the whole proximal row of tarsalia.

The pes is generally pentedactylate, but in some Crocodiles the fifth
digit is vestigial (fig. 48, B), and in some Dinosauria (fig. 35)
there are only three digits. The North American Dinosaurs present a
continuous series ranging from a pentedactylate plantigrade form like
_Morosaurus_, to such a form as _Hallopus_ with a highly digitigrade
and specialised pes reduced to three functional digits, and a
vestigial fifth metatarsal. The second, third and fourth metatarsals
in this form are nearly two-thirds as long as the femur, and the
calcaneum is drawn out into a heel much as it is in most mammals.

In Lacertilia, Orthopoda and many Chelonia, the ankle joint comes to
lie between the proximal and distal row of tarsals as in birds.

FOOTNOTES:

[91] See pp. 214 and 215.

[92] These terms are defined on p. 199.

[93] E.T. Newton, _Phil. Trans._ vol. CLXXXIV, B, p. 431 (1893).

CHAPTER XVII.

CLASS. AVES[94].

Birds form a large and extremely homogeneous class of the vertebrata,
and are readily distinguished from all other animals by the possession
of an epidermal exoskeleton having the form of feathers. Feathers
differ from hairs in the fact that they grow from papillae formed of
both the horny and the Malpighian layer of the epidermis, which
papillae at first project from the surface, and only subsequently
become imbedded in pits of the dermis. A dermal exoskeleton does not
occur in birds.

The endoskeleton is characterised by its lightness, the large bones
being generally hollow; but the pneumaticity does not vary in
proportion to the power of flight. The cervical part of the vertebral
column is very long and flexible, while the post-cervical portion is
generally very rigid, owing to the fusion of many of the vertebrae,
especially in the lumbar and sacral regions. The vertebrae are
generally without epiphyses to their centra. The cervical vertebrae in
living forms have saddle-shaped articulating surfaces, and many of
them bear ribs. The thoracic ribs in almost all birds have large
uncinate processes. The sternum is very large, and the ribs are always
attached to its sides, not as in many reptiles to anybackwardly
projecting process or processes. The sternum ossifies from two or more
centres.

The skull is extremely light, and its component bones show a great
tendency to fuse together completely. The facial part of the skull is
prolonged into a beak, chiefly formed of the premaxillae; this beak is
in all modern birds devoid of teeth, and is coated externally with a
horny epidermal sheath. The quadrate is large and freely movable. The
supratemporal arcade[1] is imperfect, while the infratemporal
arcade[95] is complete. There are no postorbital or postfrontal bones.
Neither parotic processes nor an interparietal foramen occur. There
are commonly large pre-orbital vacuities. The palatines and pterygoids
never form a secondary bony palate as in Crocodiles. Part of the floor
of the skull is formed by a wide _basitemporal_ (paired in the embryo)
which is continued in front as a long slender _rostrum_; these
structures have replaced the parasphenoid of Ichthyopsids. Cartilage
or bone is always developed in the sclerotic. The first branchial arch
is well developed, the hyoid arch but slightly. The coracoids are
large, and the clavicles are nearly always united forming the
_furcula_. There is no separate interclavicle and hardly any trace of
a precoracoid.

The anterior limbs form wings, and the manus is in the adult always
much modified, never having more than three digits. The three bones of
the pelvis are, except in Archaeornithes, always ankylosed together in
the adult, and the ilium is greatly prolonged in front of the
acetabulum, which is perforated. The ilia are not connected with the
sacrum by ossified sacral ribs. The pubes and ischia are directed
backwards parallel to one another, and except in a very few forms
never meet their fellows in ventral symphyses. The fibula is generally
much reduced. The proximal tarsal bones are always ankylosed to the
tibia, and the distal tarsals to the metatarsals, so that the ankle
joint is _intertarsal_. The first metatarsal is nearly always free.
The pes never has more than four digits in the adult.

The class Aves is most conveniently divided into two subclasses: 1.
Archaeornithes. 2. Neornithes.

Subclass I. ARCHAEORNITHES.

The only form referred to this subclass of extinct birds is
_Archaeopteryx_[96], the earliest known bird. In this animal the
skeleton does not seem to be pneumatic. The cervical and trunk
vertebrae are generally thought to be flat, certainly their
articulating surfaces are not saddle-shaped. There is no long compound
sacrum as in modern birds. The tail is longer than the whole body, the
caudal vertebrae are twenty in number, they gradually taper as traced
away from the trunk, and each bears a pair of feathers. The posterior
caudal vertebrae are not united together to form a _pygostyle_. The
upper jaw bears thirteen pairs of conical teeth, planted in distinct
sockets in the maxillae and premaxillae, but the mandible has only three
pairs. The presence of these teeth forms the most essential difference
between the skull of _Archaeopteryx_ and that of modern birds, and the
fact that they occur on the premaxillae renders it improbable that a
horny beak was present. There is a ring of ossifications in the
sclerotic. The ribs do not show uncinate processes, and articulate
with the vertebrae by single heads not divided into capitula and
tubercula. Abdominal ribs appear to have been present. The furcula is
large, and the scapula has a well developed acromion. The sternum is
unknown. The radius and ulna are approximately equal in size. In the
manus the first, second and third digits[97] are present, each
terminated by a claw. The second digit is considerably the longest,
while the third includes four phalanges. The three bones of the pelvis
probably remained distinct throughout life. The tarsals are ankylosed
respectively to the tibia and metatarsals as in other birds. The
metatarsals are ankylosed together, and the pes has four digits.

_Subclass II._ NEORNITHES.

To this subclass may be referred all known birds except
_Archaeopteryx_. They all agree in having a short tail whose component
vertebrae are commonly ankylosed together forming a pygostyle. The
three metacarpals do not all remain distinct. The bones of the pelvis
are ankylosed together, and to a large though variable number of
vertebrae. There are three orders, the Ratitae, Odontolcae, and
Carinatae.

_Order_ 1. RATITAE.

The Ratitae differ from _Archaeopteryx_ and the great majority of
Carinatae in being flightless. The bones are generally not pneumatic,
containing marrow instead of air, in the Ostrich however they are very
pneumatic. The tail is short and the posterior caudal vertebrae are
generally ankylosed together forming a pygostyle. The pectoral girdle
has comparatively a much smaller size than in Carinatae, clavicles are
small or absent, and the scapula and coracoid lie nearly in the same
straight line. The ilium and ischium do not as in Carinatae unite
posteriorly, and enclose a foramen except in very old Rheas and Emeus.
The quadrate articulates with the cranium by a single head. The vomers
unite and form a broad plate, separating the palatines, pterygoids and
basisphenoidal rostrum.

The anterior limbs are greatly reduced in size or even absent, while
the posterior limbs are greatly developed and adapted for running. The
tibia and fibula are quite distinct.

Many ornithologists agree that the various forms grouped together as
Ratitae are not all very closely allied to one another, that they
resemble one another mainly in having lost the power of flight, and do
not form a natural group.

The Ratitae include the following groups:--

_Æpyornithes_[98], huge extinct birds from Madagascar.

_Apteryges_, including the Apteryx of New Zealand.

_Dinornithes_[99], the Moas, huge extinct birds from New Zealand, and
some of the neighbouring islands.

_Megistanes_, including the Cassowaries (_Casuarius_) of Australia,
New Guinea, and some of the neighbouring islands; and the Emeus
(_Dromaeus_) of Australia.

_Rheornithes_, including the Rheas of S. America.

_Struthiornithes_, including the Ostriches (_Struthio_) now living in
Africa, and found fossil in N. India and Samos.

_Order_ 2. ODONTOLCAE.

This order includes only an extinct N. American bird
_Hesperornis_[100]. The jaws are provided with a series of sharp teeth
placed in continuous grooves, but the premaxillae are toothless, and
were probably sheathed in a horny beak. The rami of the mandible are
not ankylosed together in front. The skeleton is not pneumatic. The
cervical vertebrae have saddle-shaped articulating surfaces as in
ordinary birds, and the thoracic vertebrae are not ankylosed together.
The tail is comparatively long, and formed of twelve vertebrae with
only slight indications of a pygostyle. The ribs have uncinate
processes. The anterior limb is quite vestigial, being reduced to a
slender humerus. The posterior limb is very powerful and adapted for
swimming.

_Order_ 3. CARINATAE.

This order includes the vast majority of living birds. The cervical
vertebrae have saddle-shaped articulating surfaces (except in the
Ichthyornithiformes). The posterior caudal vertebrae are ankylosed
forming a pygostyle. The quadrate articulates with the cranium by a
double head. In all except the Tinamidae the vomers are narrow behind
and not interposed between the palatines, pterygoids and
basisphenoidal rostrum. The sternum has a median keel, and the
anterior limbs are in the great majority of cases adapted for flight.
Clavicles are well developed, and the scapula and coracoid are nearly
at right angles to one another. The various groups into which the
Carinatae are divisible are shown in the table on pp. 40-42. Their
special characters will not be dealt with.

[Illustration FIG. 55. _Gallus bankiva_ var. _domesticus_. THE LEFT
HALF OF THE SKELETON. The skull, vertebral column, and sternum are
bisected in the median plane. (After Marshall and Hurst.)

A, acetabulum. B, cerebral fossa. CB, cerebellar fossa. CL, clavicle.
CO, coracoid. CR, cervical rib. C 1 = one, first cervical vertebra.
FE, femur. HC, ventral end of clavicle. HU, humerus. HY, hyoid. IF,
ilio-sciatic foramen. IL, ilium. IS, ischium. L, lachrymal. MC 3,
postaxial metacarpal. MN, mandible. MS, xiphoid processes. MT,
tarso-metatarsus. MT 1, first metatarsal. N, nasal. OP, optic foramen.
P, premaxillae. PB, pubis. PL, palatine. PY, pygostyle. R, radius. RC,
radial carpal. S, keel of sternum. SC, scapula. T, tibio-tarsus. TH 4,
fourth thoracic vertebra. U, ulna. UC, ulnar carpal. UP, uncinate
process. Z, infra-orbital bar. 1, 2, 3, 4, first, second, third and
fourth digits of pes. 3, pre-axial, 4, middle, and 5, postaxial digit
of manus.]

FOOTNOTES:

[94] M. Fürbringer, _Untersuchungen zur Morphologie und Systematik der
Vögel_, I. and II. Amsterdam, 1888. Cf. H. Gadow, _Nature_, XXXIX.
1888, pp. 150 and 177.

T.H. Huxley, "On the classification of birds." _P.Z.S._, London, 1867.

E. Selenka and H. Gadow, _Vögel_ in Bronn's _Classen und Ordnungen des
Thierreichs_ 1869-1890.

[95] See p. 283.

[96] R. Owen, _Phil. Trans._, vol. CLIII., p. 33; 1863. T.H. Huxley,
_P. R.S._, vol. XVI., p. 243; 1868. C. Vogt, _Rev. Scient._, ser. 2,
tom. 9, p. 241; 1879. C.H. Hurst, _Nat. Sci._, vol. III., p. 275;
1893; vol. VI., pp. 112, 180, 244; 1895. W.P. Pycraft, _Nat. Sci._,
vol. V., pp. 350 and 437; 1894; and vol. VIII., p. 261; 1896.

[97] According to Hurst the fourth and fifth digits are also present.

[98] See C.W. Andrews, _P.Z.S._, 1894, p. 108.

[99] See T.J. Parker, _Tr. Zool. Soc. London_, vol. XIII., pt. 2,
1895, and F.W. Hutton, several papers in _Tr. N. Zealand Inst._, 1893
and 1895.

[100] See O.C. Marsh. _Odontornithes. A monograph of the extinct
toothed birds of N. America._ New Haven, 1880.

CHAPTER XVIII.

THE SKELETON OF THE WILD DUCK (_Anas boschas_).

I. EXOSKELETON.

The exoskeleton of the Duck and indeed of all birds is entirely
epidermal in origin. Its most important part consists of =feathers=,
but it includes also the following horny structures:--

(_a_) =scales=, which cover the toes and tarso-metatarsus;

(_b_) =claws=, which are attached to the distal phalanges of the toes
and of the pollex;

(_c_) the wide =beak=, which sheaths both upper and lower jaws, and
whose edges are raised into lamellae, which act as strainers.

FEATHERS.

A well developed feather, such as one of the large quill feathers of
the wing or tail, consists of the following parts: A main stem, the
=scapus=, which forms the axis running along the whole length of the
feather, and is divided into (1) a proximal hollow cylindrical
portion, the =calamus= or =quill=, and (2) a distal solid portion, the
=rachis= or =shaft=, which is square in section, flexible and grooved
along its ventral surface, and bears a number of lateral processes,
the =barbs=. The =calamus= which is partly imbedded in a pit in the
dermis, bears two holes: one, the =inferior umbilicus=, is at its
proximal end, and into it enters a vascular outgrowth from the dermis;
the other, the =superior umbilicus=, lies on the ventral surface at
the junction of the calamus and scapus.

The =barbs= are a series of narrow elastic plates, attached by their
bases to the rachis, and with their edges looking upwards and
downwards. The barbs are connected together by a number of smaller
processes, the =barbules=, which interlock with one another by means
of hooklets, and bear the same relation to the barbs that the barbs do
to the rachis. The barbs and barbules, together with the rachis,
constitute the =vexillum= or =vane= of the feather. Any feather having
the above type of structure is called a =penna= or a =contour
feather=, from the fact that it helps to produce the contour of the
body.

VARIETIES OF FEATHERS.

1. =Pennae.= There are two kinds of pennae or contour feathers.

(_a_) The =quills=. These form the large feathers of the wing and
tail. They are divided into two groups, the =remiges=, or wing quills,
and the =rectrices=, or tail quills.

The =remiges=[101] include three sets of feathers, the =primaries= or
=metacarpo-digitals=, which are attached to the bones of the manus,
the =secondaries= or =cubitals=, which are attached to the ulna, and
the =humerals=, which are attached to the humerus.

The =primaries= differ from all the other quill feathers in having the
posterior half of the vane much wider than the anterior half. They are
ten in number, and of these six, the =metacarpal= quills (fig. 57,
14), are attached to the second and third metacarpals, one, the
=ad-digital= (fig. 57, 15), to the phalanx of the third digit, two,
the =mid-digitals= (fig. 57, 16), to the first phalanx of the second
digit, and two, the =pre-digitals= (fig. 51, 17), to the second
phalanx of the second digit. One of the pre-digitals is very small,
and is called the =remicle= (fig. 57, 11).

[Illustration FIG. 56. THE WING OF A WILD DUCK (_Anas boschas_).

The upper figure shows the dorsal side of a right wing, the lower
figure the ventral side of a left wing. × 1/3. (Brit. Mus.)

    1. scapulars.
    2. tectrices marginales.
    3. tectrices minores.
    4. bastard wing.
    5. tectrices majores.
    6. metacarpo-digitals or primaries.
    7. tectrices mediae.
    8. cubitals or secondaries.
    9. pennae humerales.
    10. pennae axillares.]

In addition, a group of three quill feathers is attached to the first
digit, constituting the =bastard wing= or =ala spuria= (fig. 56, 4).

The =secondaries= or =cubitals= (fig. 56, 8) form a group of seventeen
feathers, attached to the ulna; they are shorter than the primaries,
and do not have the posterior half of the vane much wider than the
anterior half.

The =humerals= (figs. 56, 9 and 57, 12) form a group of eight small
feathers, of varying length, attached to the anterior half of the
humerus.

[Illustration FIG. 57. WINGS OF A WILD DUCK WITH THE COVERTS REMOVED
(_Anas boschas_). × 1/3.

A. Right wing seen from the dorsal side. B. Left wing disarticulated
and seen from the ventral side. (Brit. Mus.)

    1. humerus.
    2. radius.
    3. ulna.
    4. radial carpal.
    5. ulnar carpal.
    6. first phalanx of first digit.
    7. second metacarpal.
    8. third metacarpal.
    9. first phalanx of second digit.
    10. second phalanx of second digit.
    11. remicle.
    12. pennae humerales.
    13. cubitals or secondaries.
    14. metacarpal quills.
    15. ad-digital.
    16. mid-digitals.
    17. pre-digital.]

(b) The =tectrices= or =coverts= are short feathers, which cover over
the quills of the rectrices and remiges, and clothe the body
generally. Their barbules are less developed than is the case with the
quill feathers, so that the barbs separate readily from one another,
especially at the base of the vane. The nomenclature of the various
patches of coverts on the wings is seen in fig. 56. A small patch of
backwardly-directed feathers surrounding the external auditory opening
are known as the =auriculars=.

2. The =filoplumes= are rudimentary feathers, consisting of a minute
stem and slightly developed vane. They are left in the skin after the
other feathers have been removed.

3. The =plumulae=, or down feathers, have the stem very slightly
developed, while the barbs are soft and free from one another. They
are distributed all over the body, not only among the contour
feathers, but also over the spaces (_apteria_) which bear no contour
feathers.

In the young bird the rudiments of the new feathers are formed at the
bases of the embryonic down feathers, and as they grow they push them
out from the skin. The embryonic down feathers however remain attached
to the apices of the new feathers till these have reached a length of
about an inch; they are then shed.

II. ENDOSKELETON.

As compared with that of the Turtle or Crocodile, the endoskeleton of
the Duck is characterised by:

1. The great lightness of the bones, many of which contain air
cavities.

2. The tendency to become ankylosed together shown by many of the
bones.

3. The modification of the anterior limbs and girdle for the purpose
of flight.

1. THE AXIAL SKELETON.

This, as in other vertebrates, is divisible into--

    A. The vertebral column.

    B. The skull.

    C. The ribs and sternum.

A. THE VERTEBRAL COLUMN.

The vertebral column of the duck, like that of the great majority of
birds, presents a number of well-marked characteristics, contrasting
strongly with those of the generality of higher vertebrates. The centra