CHAPTER XIV.

THE SKELETON OF THE GREEN TURTLE.

(_Chelone midas._)

The most striking feature as regards the skeleton of the Turtle is that the trunk is enveloped in a bony box, the dorsal portion of which is called the =carapace=, while the ventral portion is the =plastron=.

I. EXOSKELETON.

_a._ The =epidermal exoskeleton= in the Green Turtle as in all other Chelonia except _Dermochelys_, _Trionyx_ and their allies is strongly developed, its most important part consisting of a series of horny =shields= which cover over the bony plates of the carapace and plastron but do not at all correspond to them in size and arrangement.

The shields covering over the =carapace= consist of three rows of larger central shields,--five (=vertebral=) shields being included in the middle row and four (=costal=) in each lateral row,--and of a number of smaller =marginal= shields.

Of the marginal shields, that lying immediately in front of the first vertebral is termed the =nuchal=, while the two succeeding the last vertebral are called sometimes =pygal=, sometimes =supracaudal=; the remainder are the marginal shields proper.

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.

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.