There is no separate precoracoid but a precoracoidal process (fig. 34, 7) of the coracoid is generally prominent.

Sternal ribs are present in chamaeleons and scinks. The limbs are in the great majority of cases pentedactylate and the digits are clawed. The phalanges articulate by means of condyles. Sometimes one or both pairs of limbs are absent. When the posterior limbs are absent the pelvis is also wanting, though the loss of the anterior limbs does not lead to a corresponding loss of the pectoral girdle.

The pubis corresponds to the pre-pubis of Dinosaurs, and both pubes and ischia meet in ventral symphyses.

The suborder includes the Lizards, Chamaeleons and Amphisbaenians.

Suborder (2). Ophidia[81].

The Ophidia or snakes are characterised by their greatly elongated body and want of limbs. The body is covered with overlapping horny scales and bony dermal scutes are never present. The vertebrae are procoelous, and are distinguishable into two groups only, precaudal or rib-bearing, and caudal or ribless. The atlas vertebra is also ribless. The neural arches are always provided with zygosphenes and zygantra. Many of the vertebrae have strong hypapophyses, and the caudal vertebrae are without chevron bones.

In the skull the cranial cavity extends forwards between the orbits, and is closed in front by downgrowths from the frontals and parietals which meet the well-ossified alisphenoids and orbitosphenoids[82]. The cranium is strongly ossified, and there are no parotic processes or interparietal foramen. There are no temporal arcades and no epipterygoid. The premaxillae if present are very small (fig. 51, 1) and usually toothless. The quadrates articulate with the squamosals, and do not as in Lacertilia meet the exoccipitals. The palatines do not unite directly with the vomers or with the base of the cranium, and the whole palato-maxillary apparatus is more loosely connected with the cranium than it is in Lacertilia. The pterygoids, and in most cases also the palatines, bear teeth. The dentition is acrodont, and the rami of the mandible are united only by an elastic ligament—an important point serving to distinguish the Ophidia from the Lacertilia. There is an imperfectly developed interorbital septum, the ventral part of which is formed by the parasphenoid. The postfrontal is generally well developed, while the jugals and quadratojugals are absent. There are never any traces of the anterior limbs or pectoral girdle, but occasionally there are vestiges of a pelvis and posterior limbs.

Suborder (3). Pythonomorpha[83].

This suborder includes Mosasaurus and its allies, a group of enormous extinct marine reptiles found in beds of Cretaceous age.

The skin is in most forms at any rate unprovided with dermal scutes. The vertebrae may be with or without zygosphenes and zygantra. The skull resembles that of lizards, having an interparietal foramen, and a cranial cavity open in front. The squamosal takes part in the formation of the cranial wall, and the quadrate articulates with the squamosal, not as in Lacertilia with the exoccipital. There are large supratemporal fossae, bounded below by supratemporal arcades. The teeth are large and acrodont, and occur on the pterygoids as well as on the jaws. The two rami of the mandible are united by ligament only. Pectoral and pelvic girdles are present, but clavicles are wanting, and the pelvis is not as a rule united to any sacrum.

The limbs are pentedactylate, and are adapted for swimming, while all the limb bones except the phalanges are relatively very short. The number of phalanges is not increased beyond the normal, and they articulate with one another by flat surfaces. The terminal phalanges are without claws.

Order 7. Dinosauria[84].

The extinct reptiles comprising this order were all terrestrial, and include the largest terrestrial animals known. They vary greatly in size and in the structure of the limbs, some approach close to the type of structure met with in birds, others are allied to crocodiles.

Passing to the more detailed characters:—there is sometimes a well-developed exoskeleton having the form of bony plates or spines. The vertebrae may be solid or their centra may be hollowed internally; their surfaces may be flat, biconcave or opisthocoelous. The sacrum is composed of from two to six vertebrae.

As regards the skull, the quadrate is large and fixed, and supratemporal and infratemporal fossae bounded by bone occur. The teeth are more or less laterally compressed, and often have serrated edges; they may be placed in distinct sockets or in a continuous groove. The ribs have capitula and tubercula, and sternal ribs often occur. The scapula is very large, the coracoid small, and there is no precoracoid, or T-shaped interclavicle. Clavicles are only known in a few cases. In the pelvis the ilium is elongated both in front of, and behind, the acetabulum, sometimes the pre-pubis, sometimes the post-pubis is the better developed. The anterior limbs are shorter than the posterior, and the long bones are sometimes solid, sometimes hollow.

There are three well-marked suborders of the Dinosauria.

Suborder (1). Sauropoda[85].

The reptiles belonging to this group were probably quadrupedal and herbivorous.

They have the cervical and anterior trunk vertebrae opisthocoelous, while the posterior vertebrae are biconcave; all the presacral, and sometimes the sacral vertebrae are hollowed internally. The teeth are spatulate and without serrated edges, they are always planted in distinct sockets, and some of them are borne by the premaxillae.

The nares have the form of long slits and there are large pre-orbital vacuities.

The limb bones are solid, and the anterior limbs are not much shorter than the posterior ones. All the limbs are plantigrade and pentedactylate, and the digits of the pes are clawed. There is a large pre-pubis directed downwards and forwards, meeting its fellow in a ventral symphysis, but there is no post-pubis.

The Sauropoda are found in the secondary rocks of Europe and N. America and include the largest land animals that are known to have existed. Many of the best known forms such as Brontosaurus and Morosaurus are North American.

Suborder (2). Theropoda.

The members of this suborder were all carnivorous, and from the small comparative size of the anterior limbs many of them were probably bipedal.

The vertebrae are opisthocoelous or amphicoelous, their neural arches are provided with zygosphenes and zygantra, and their centra are frequently hollowed internally; the limb bones are also hollow, and in fact the whole skeleton is extremely light. The tail is of great length. The teeth are pointed and recurved, and have one or both borders serrated; they are always planted in distinct sockets, and some of them are borne by the premaxillae. There are large pre-orbital vacuities. The digits of both manus and pes are terminated by pointed ungual phalanges which must have borne claws. In the pelvis the pre-pubes and ischia are slender bones, the former meeting in a ventral symphysis. The ilia are very deep vertically and there are no post-pubes. The astragalus is closely applied to the tibia, in front of which it sends an ascending process, sometimes the two bones appear to have been ankylosed together, as in birds. The metatarsals are elongated and the feet digitigrade.

The Theropoda vary greatly in size, one of the best known genera Compsognathus was about as large as a cat, another, Megalosaurus, perhaps as large as an elephant. Ceratosaurus is the name of a well-known North American form regarded by many authorities as identical with Megalosaurus.

Suborder (3). Orthopoda.

This suborder includes the most specialised of the Dinosaurs, certain of which resemble the Theropoda in being bipedal. In some of them such as Stegosaurus the exoskeleton is strongly developed, in others such as Iguanodon it is absent.

The vertebrae are solid and may be opisthocoelous, biconcave, or flat. The teeth are compressed and serrated, often irregularly, and are frequently not set in distinct sockets. The anterior part of the premaxillae is without teeth, and a toothless predentary or mento-meckelian bone is present. The pre-orbital vacuities are small or absent, and the nares are large and placed far forwards.

The most characteristic features of the group are found in the pelvis which, except in the Ceratopsia, bears a striking resemblance to that of birds. The ischium and post-pubis are long slender bones directed backwards parallel to one another, and the pre-pubis is also well developed. The ischium has an obturator process. The limb bones are sometimes hollow, sometimes solid. The anterior limbs are much shorter than the posterior, pointing to a bipedal method of progression. The pes is digitigrade or plantigrade, and has three, rarely four, digits.

The suborder Orthopoda may be further subdivided into three sections:—

A. Stegosauria.

A dermal exoskeleton is strongly developed. The vertebral centra are flat or biconcave, and neither they nor the limb bones are hollowed out by internal cavities. The limbs are plantigrade, the anterior ones short, the posterior ones very large and strong. The post-pubis is well developed;

e.g. Stegosaurus from the Upper Jurassic of Colorado.

B. Ceratopsia.

There is sometimes a well-developed dermal exoskeleton formed of small granules and plates of bone. The bones are solid, and the vertebral centra flat. The cranium bears a pair of enormous pointed frontal horns, and the parietal is greatly expanded and elevated behind, forming with the squamosals a shield which overhangs the anterior cervical vertebrae. The premaxillae are united, and in front of them is a pointed beak-like bone which bites upon a toothless predentary ossification of the mandible. The teeth have two roots. The anterior limbs are but little shorter than the posterior ones. There is no post-pubis;

e.g. Polyonax from the uppermost Cretaceous of Montana.

C. Ornithopoda[86].

There is no dermal exoskeleton. The cervical vertebrae are opisthocoelous, and so are sometimes the thoracic. The limb bones are hollow and the anterior limbs are much shorter than the posterior ones. The feet are digitigrade and provided with long pointed claws. The post-pubis is long and slender and directed back parallel to the ischium;

e.g. Iguanodon from the European Cretaceous.

Order 8. Crocodilia[87].

This order includes the Crocodiles, Alligators and Garials and various extinct forms, some of which are closely allied to the early Dinosaurs.

There is always a more or less complete exoskeleton formed of bony scutes overlain by epidermal scales; these bony scutes are specially well developed on the dorsal surface but may occur also on the ventral. The vertebral column is divisible into the five regions commonly distinguishable. In all living forms the vertebrae, with the exception of the atlas and axis, the two sacrals, and first caudal, are procoelous, but in many extinct forms they are amphicoelous. The atlas (fig. 71) is remarkable, consisting of four pieces, and the first caudal is biconvex.

The teeth are, in the adult, planted in separate deep sockets. The skull is very dense and solid, and all the component bones including the quadrate are firmly united. The dorsal surface of the skull is generally characteristically sculptured. There is an interorbital septum, and the orbitosphenoidal and presphenoidal regions are imperfectly ossified. Supratemporal, infratemporal, and post-temporal fossae occur, but no interparietal foramen. In living genera there is a long secondary palate formed by the meeting in the middle line of the palatines, pterygoids and maxillae (fig. 43, A).

Cervical ribs (fig. 41, 8 and 9) are well developed, and articulate with rather prominent surfaces borne on the neural arches and centra respectively. The thoracic ribs articulate with the long transverse processes, and sternal ribs and abdominal splint ribs (fig. 46, 4) occur. The sternum is cartilaginous, and both it and the shoulder-girdle are very simple. The precoracoid is represented by merely a small process on the coracoid, while the clavicles are absent, except in the Parasuchia. In the pelvis (fig. 49) there is a large ilium, and an ischium meeting its fellow in a ventral symphysis; these two bones form almost the whole of the acetabulum. In front of the acetabulum, in the Eusuchia, projects a bone which is generally called the pubis, but is in reality rather an epipubis (fig. 49, 4), the true pubis being probably represented by a fourth element which remains cartilaginous for some time, and later on ossifies and attaches itself to the ischium. The limbs are small in proportion to the size of the body, and are adapted for swimming or for shuffling along the ground; they are plantigrade and the bones are all solid. In living forms the anterior limbs have five digits and the posterior four, the fifth being represented only by a short metatarsal. The first three digits in each case are clawed. The calcaneum has a large backwardly-projecting process.

The order Crocodilia may be subdivided into two suborders.

Suborder (1). Parasuchia.

The vertebral centra are flat or biconcave. The premaxillae are very large, and the nares are separated, and placed far back. The posterior narial openings lie comparatively far forward between the anterior extremities of the palatines.

The palatines and pterygoids do not form a secondary palate. The supratemporal fossae are small, and open posteriorly, the lateral temporal fossae are very large. The parietals and frontals are paired. Clavicles are present. The best known and most important genus of these extinct crocodiles is Belodon.

Suborder (2). Eusuchia.

The vertebrae are either biconcave or procoelous. The premaxillae are small, and the anterior nares are united and placed far forwards. The posterior nares lie far back, the palatines and in living genera the pterygoids, meeting in the middle line, and giving rise to a closed palate. The supratemporal fossae are surrounded by bone on all sides, and the parietals, and often also the frontals are united. There are no clavicles. The suborder includes the genera Crocodilus, Alligator, Garialis and others living and extinct.

Order 9. Pterosauria[88].

These animals, called also the pterodactyles or Ornithosauria, are a group of extinct reptiles, whose structure has been greatly modified from the ordinary reptilian type for the purpose of flight.

The skin was naked and they vary greatly in size and in the length of the tail. The vertebrae and limb bones are pneumatic just as in birds. The presacral vertebrae are procoelous and have their neural arches firmly united to the centra. The neck is long, the caudal vertebrae are amphicoelous, and from three to five vertebrae are fused together in the sacral region. The skull is large and somewhat bird-like, the facial portion being much drawn out anteriorly, and the sutures being obliterated. It resembles that of other reptiles in having large supratemporal fossae; large pre-orbital vacuities also occur. The jaws may be toothed or toothless, and the teeth, when present, are imbedded in separate sockets. The premaxillae are large, and the quadrate is firmly attached to the skull. The rami of the mandible are united at the symphysis, and there is an ossified ring in the sclerotic. The occurrence of a postfrontal and its union with the jugal behind the orbit, are characteristic reptilian features.

The ribs have capitula and tubercula, and sternal and abdominal ribs occur. The sternum has a well-developed keel, and the scapula and coracoid are large and bird-like. There are no clavicles or interclavicle.

The anterior limbs are modified to form wings by the great elongation of the fifth digit, to which a membrane was attached. The second, third and fourth digits are clawed and are not elongated in the way that they are in bats. The pollex, if present at all, is quite vestigial.

The pelvis is weak and small, and though the ilia are produced both in front of and behind the acetabula, in other features the pelvis is not bird-like. The ischia are short and wide, and the pubes are represented only by the pre-pubes. The posterior limbs are small and the fibula is much reduced. The pes is quite reptilian in type, and has five separate slender metatarsals. The two best known genera are Pterodactylus, in which the tail is short, and Rhamphorhynchus, in which it is long. The Pterosauria are found throughout the Jurassic and Cretaceous formations in both Europe and North America.

CHAPTER XIV.
THE SKELETON OF THE GREEN TURTLE. (Chelone midas.)

The epidermal covering of the plastron consists principally of six pairs of symmetrically arranged shields, called respectively the gular, humeral, pectoral, abdominal, femoral, and anal, the gular being the most anterior. In front of the gular shields is an unpaired intergular, and the shields of the plastron are connected laterally with those of the carapace, by five or six pairs of rather irregular infra-marginal shields. Smaller horny plates occur on other parts of the body, especially on the limbs and head.

Two other sets of structures belong also to the epidermal exoskeleton, viz. (a) horny beaks with denticulated edges which ensheath both upper and lower jaws, (b) claws, which as a rule are borne only by the first digit of each limb. Sometimes in young individuals the second digit is also clawed.

b. The dermal exoskeleton is strongly developed, and is combined with endoskeletal structures derived from the ribs and vertebrae to form the carapace.

The Carapace (fig. 36) consists of a number of plates firmly united to one another by sutures. They have a very definite arrangement and include:

(a) the nuchal plate (fig. 36, 1), a wide plate forming the whole of the anterior margin of the carapace. It is succeeded by three series of plates, eight in each series, which together make up the main part of the carapace. Of these the small

(b) neural plates[1] (fig. 36, A, 2) form the middle series. They are closely united with the neural arches of the underlying vertebrae;

(c) the costal plates[89] (fig. 36, A, 3) are broad arched plates united to one another by long straight sutures. They are united at their inner extremities with the neural plates, but the boundaries of the two sets of plates do not regularly correspond. Each is united ventrally with a rib which projects beyond it laterally for some distance; (d) the marginal plates (fig. 36, 4) are twenty-three in number, eleven lying on each side, while an unpaired one lies in the middle line posteriorly. Many of them are marked by slight depressions into which the ends of the ribs fit; (e) the pygal plates (fig. 36, 5) are two unpaired plates lying immediately posterior to the last neural.

The sculpturing due to the epidermal shields is very obvious on the carapace.

The plastron (fig. 37) consists of one unpaired ossification, the entoplastron, and four pairs of ossifications called respectively the epiplastra, hyoplastra, hypoplastra, and xiphiplastra.

The epiplastra (fig. 37, 1) are the most anterior, they are expanded and united to one another in the middle line in front, while behind each tapers to a point which lies external to a process projecting forwards from the hyoplastron. They are homologous with the clavicles of other vertebrates.

The entoplastron or episternum (fig. 37, 2) which is homologous with the interclavicle of other reptiles, is expanded at its anterior end and attached to the symphysis of the epiplastra, while behind it tapers to a point and ends freely.

The hyoplastra are large irregular bones each closely united posteriorly with the corresponding hypoplastron, and drawn out anteriorly into a process which lies internal to that projecting backwards from the epiplastron. Each gives off on its inner surface a slender process which nearly meets its fellow, while the anterior half of the outer surface is drawn out into several diverging processes.

The hypoplastra (fig. 37, 4) are flattened bones resembling the hyoplastra, with which they are united by long sutures; the posterior half of both outer and inner surfaces is drawn out into a number of pointed processes.

The xiphiplastra are small flattened elongated bones meeting one another in the middle line posteriorly. In front they are notched and each interlocks with a process from the hypoplastron of its side. The hyoplastra, hypoplastra and xiphiplastra are homologous with the abdominal ribs of Crocodiles.

II. ENDOSKELETON.

1. The Axial Skeleton.

The axial skeleton includes the vertebral column, the ribs, and the skull.

A. The Vertebral column and Ribs.

The number of vertebrae in the Green Turtle is thirty-eight, not a great number as compared with that in many reptiles, and of these eighteen are caudal.

The vertebral column is divisible into four regions only—cervical, thoracic, sacral, and caudal.

The Cervical vertebrae.

These are eight in number, and are chiefly remarkable for the great variety of articulating surfaces which their centra present, and for their mobility upon one another.

The first or atlas vertebra differs much from all the others and consists of the following parts:—

a. the neural arch, formed of two separate ossifications united in the mid-dorsal line;

b. the inferior arch;

c. the centrum, which is detached from the rest and forms the odontoid process of the second vertebra.

Each half of the neural arch consists of a ventral portion, the pedicel, which lies more or less vertically and is united ventrally to the inferior arch, and of a dorsal portion, the lamina, which lies more or less horizontally and meets its fellow in the middle line in front, partially roofing over the neural canal. Each pedicel bears a facet on its anterior surface, which, with a corresponding one on the inferior arch, articulates with the occipital condyle of the skull. Three similar facets occur also on the posterior surface of the pedicel and inferior arch, and articulate with the odontoid process. The laminae meet one another in front, but do not fuse, while behind they are separated by a wide triangular space. They bear a pair of small downwardly-directed facets, the postzygapophyses, for articulation with the prezygapophyses of the second vertebra.

The inferior arch is a short irregular bone bearing two converging facets for articulation with the occipital condyle and odontoid process respectively.

The centrum or odontoid process has a convex anterior surface for articulation with the neural and inferior arches, and a concave posterior surface by which it is united with the centrum of the second or axis vertebra. It bears posteriorly a small epiphysis which is really a detached portion of the inferior arch.

The second or axis and following five cervical vertebrae, though showing distinct differences, resemble one another considerably, each having a fairly elongated centrum with a keel-like hypapophysis, each having also a neural arch with prominent articulating surfaces, the anterior of which, or prezygapophyses, look upwards and inwards, while the posterior ones, the postzygapophyses, look downwards and outwards. They however, as was previously mentioned, differ very remarkably in the character of the articulating surfaces of the centra. Thus the second and third vertebrae are convex in front and concave behind, the fourth is biconvex, the fifth is concave in front and convex behind. The sixth is concave in front and attached to the seventh by a flat surface behind, the seventh has a flat anterior face and two slightly convex facets behind. The vertebrae all have short blunt transverse processes and the second has a prominent neural spine.

The eighth cervical vertebra is curiously modified, the centrum is very short, has a rather prominent hypapophysis, and is convex behind, while in front it articulates with the preceding centrum by two concave surfaces. The neural arch is deeply notched in front and bears two upwardly-directed prezygapophyses, while behind it is very massive and is drawn out far beyond the centrum, bearing a pair of flat postzygapophyses. The top of the neural arch almost or quite meets a blunt outgrowth from the nuchal plate.

The Thoracic vertebrae.

These are ten in number and are all firmly united with the ribs and elements forming the carapace.

The first thoracic vertebra differs from the others, the centrum is short and has a concave anterior surface articulating with the centrum of the last cervical vertebra, and a pair of prezygapophyses borne on long outgrowths. The neural spine arises only from the anterior half of the centrum, and is not fused to the carapace. Arising laterally from the anterior part of the centrum are a small pair of ribs each of which is connected with a process arising from the rib of the succeeding vertebra.

The next seven thoracic vertebrae are all very similar, each has a long cylindrical centrum, expanded at the ends, and firmly united to the preceding and succeeding vertebrae. The neural arches are flattened and expanded dorsally, and are united to one another and to the overlying neural plates; each arises only from the anterior half of its respective centrum, and overlaps the centrum of the vertebra in front of it. Between the base of the neural arch and its successor is a small foramen for the exit of the spinal nerve. There are no transverse processes or zygapophyses.

To each thoracic vertebra from the second to ninth inclusive, there corresponds a pair of ribs (fig. 36, 6) of a rather special character. Each is suturally united with the anterior half of the edge of its own vertebra, and overlaps on to the posterior half of the edge of the next preceding vertebra. The ribs are much flattened, and each is fused with the corresponding costal plate, beyond which it projects to fit into a pit in one of the marginal plates.

The tenth thoracic vertebra is smaller than the others, and its neural arch does not overlap the preceding vertebra, it bears a pair of small ribs which are without costal plates, but meet those of the ninth vertebra.

There are no lumbar vertebrae.

The Sacral vertebrae.

The sacral vertebrae are two in number, they are short and wide, their centra are ankylosed together, and their neural arches are not united to the carapace.

The first has the anterior face of the centrum concave and the posterior flat, while both faces of the second are flat. Each bears a pair of short ribs which meet the ilia, but are not completely ankylosed either with them or the centra.

The Caudal vertebrae.

The caudal vertebrae are eighteen in number. The centrum of the first is flat in front and is ankylosed to the second sacral; behind it is convex. The others are all very similar to one another, and decrease gradually in size when followed back. Each has a moderately long centrum, concave in front and convex behind, both terminations being formed by epiphyses. The neural arch arises only from the anterior half of the vertebra; it bears a blunt truncated neural spine and prominent pre- and post-zygapophyses. The first seven caudal vertebrae bear short ribs attached to their lateral margins, the similar outgrowths on the succeeding vertebrae do not ossify from distinct centres, and are transverse processes rather than ribs.

B. The Skull.

The skull of the Turtle is divisible into the following three parts:—

(1) the cranium;

(2) the lower jaw or mandible;

(3) the hyoid.

(1) The Cranium.

The cranium is a very compact bony box, containing a cavity in which the brain lies, and which is a direct continuation of the neural canal of the vertebrae.

Like those of the skull as a whole its component bones may be subdivided into three sets:—

1. those forming the brain-case or cranium proper;

2. those developed in connection with the special sense organs;

3. those forming the upper jaw and suspensorial apparatus.

Both cartilage and membrane bones take part in the formation of the skull, and a considerable amount of cartilage remains unossified, especially in the ethmoidal and sphenoidal regions.

1. The Cranium proper or Brain-case.

The cartilage and membrane bones of the brain-case when taken together can be seen to be more or less arranged in three rings or segments, called respectively the occipital, parietal, and frontal segments.

The occipital segment is the most posterior of these, and consists of four cartilage bones, the basi-occipital, the two exoccipitals and the supra-occipital; these bound the foramen magnum.

The basi-occipital (figs. 38 and 39, 5) lies ventral to the foramen magnum and only bounds a very small part of it; it forms one-third of the occipital condyle by which the skull articulates with the atlas vertebra. It unites dorsally with the exoccipitals and anteriorly with the basisphenoid.

The exoccipitals are rather small bones, which form the sides and the greater part of the floor of the foramen magnum, and two-thirds of the occipital condyle. Laterally each is united with the pterygoid and opisthotic of its side. At the sides of the occipital condyle each exoccipital is pierced by a pair of foramina, the more dorsal and posterior of which transmits the hypoglossal nerve.

The supra-occipital (fig. 39, 14) is a larger bone than the others of the occipital segment. It forms the upper border of the foramen magnum and is drawn out dorsally into a large crest which extends back far beyond the occipital condyle. In the adult the supra-occipital is completely ankylosed with the epi-otics.

The Parietal segment.

The ventral portion of the parietal segment is formed by the basisphenoid (figs. 38 and 39, 4) which lies immediately in front of the basi-occipital. A triangular portion of it is seen in a ventral view of the skull, but it is quickly overlapped by the pterygoids. It gives off dorsally a pair of short processes which meet the pro-otics.

The alisphenoidal region is unossified and the only other constituents of the parietal segment are the parietals (fig. 39, 1). These are large bones which, after roofing over the cranial cavity, extend upwards and become expanded into a pair of broad plates which unite with the squamosal and bones of the frontal segment to form a wide, solid, false roof to the skull. Each also sends ventralwards a plate which meets an upgrowth from the pterygoid and acts as an alisphenoid.

The Frontal segment.

Of the frontal segment the basal or presphenoidal and lateral or orbitosphenoidal portions do not become ossified, the dorsal portion however includes three pairs of membrane bones, the frontals, prefrontals and postfrontals.

The frontals are a pair of small bones lying immediately in front of the parietals, and in front of them are the prefrontals (figs. 38 and 39, 20), a pair of similar but still smaller bones, which are produced ventrally to meet the vomer and palatines. They form also the dorsal boundary of the anterior nares. The postfrontals (figs. 38 and 39, 18) are larger bones, united dorsally to the frontals and parietals, posteriorly to the squamosals, and ventrally to the jugals and quadratojugals. All three pairs of frontal bones, especially the postfrontals, take part in the bounding of the orbits.

2. The Sense capsules.

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

The Auditory capsules.

The auditory or periotic capsule of the turtle is rather large and its walls are well ossified, epi-otic, pro-otic and opisthotic bones being present.

The epi-otic (fig. 39, 13) is the more dorsal of the three bones, and in the adult is completely ankylosed with the supra-occipital.

The opisthotic (fig. 39, 8) is the ventral posterior element. On its inner side it is united to the supra-occipital above, and to the exoccipital below; it sometimes becomes completely fused with the exoccipital. In front it meets the pro-otic, and on its outer side the squamosal and quadrate. Its anterior portion is hollowed out by the cavity in which the auditory organ lies, it gives off also a process which is separated from the exoccipital by an oval foramen through which the glossopharyngeal, pneumogastric, and spinal accessory nerves leave the cranial cavity.

The pro-otic is the anterior element; it meets the supra-occipital and opisthotic posteriorly, while anteriorly it is separated from the alisphenoidal plate of the parietal and pterygoid by a large oval foramen through which the maxillary and mandibular branches of the 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.