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.

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.

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.

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.

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.

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.

2. The Appendicular Skeleton.

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

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.

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.

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.