OSTEOLOGY OF CHELONIAN REPTILES.

213

movements in water than on land. The short limbs preclude the possibility of very quick course along shore; and the overlapping of the ribs of the neck, whilst enabling the head the better to cleave the water during the acts of diving or swimming, makes the bending of that part from side to side an act of difficulty and time; this, it is said, may avail any one pursued by a crocodile on dry land to escape by turning out of the straight course. But the crocodile usually seizes his prey by stratagem or concealment when in or close to the water; and it is there that he shows himself master of his position, and chiefly by the powerful strokes of his long, large, vertically-flattened tail. Osteology of Chelonian Reptiles-Tortoises and Turtles.-Those animals to which, in the manifold modifications of the organic framework, a portable dwelling or place of refuge has been given, in compensation for inferior powers of locomotion or other means of escape or defence, have always attracted especial attention; and of them the most remarkable, both for the complex construction of their abode as well as for their comparatively high organization, are the reptiles of the chelonian order. The expanded thoracic-abdominal case, into which, in most chelonians, the head, the tail, and the four extremities can be withdrawn, and in some of the species be there shut up by moveable doors closely fitting both the anterior and posterior apertures -as, e.g., in the box-tortoises (cinosternon, cistudo)-has been the subject of many and excellent investigations; and not the least interesting result has been the discovery that this seemingly special and anomalous superaddition to the ordinary vertebrate structure is due, in a great degree, to the modification of form and size, and, in a less degree, to a change of relative position, of ordinary elements of the vertebrate skeleton. The natural dwelling-chamber of the chelonia consists chiefly, and in the marine species (chelone) and mud-turtles (trionyx) solely, of the floor and the roof: side-walls

of variable extent are added in the fresh-water species (emydians) and land-tortoises (testudinians). The whole consists chiefly of osseous "plates" with superincumbent horny plates or "scutes," except in the soft or mud-tortoises (trionyx and sphargis), in which these latter are wanting.

214

CARAPACE OF THE TURTLE.

Fig. 20 shows the manner in which the head and tail can be retracted within the thoracic-abdominal box: the four limbs are figured as extended in the act of walking, to show their structure. The only moveable vertebræ are those of the neck and tail, and the former enjoy a great degree of flexibility. The vertebræ answering to the dorsal, lumbar, and sacral series are firmly fixed together; but the dorsal ones, 1 to 8, are chiefly concerned in the formation of the osseous dwelling-chamber. The composition of this will be first described as it exists in the turtle (chelone), the species called "loggerhead" being here selected for its illustration.

Fig. 21.

MA

m2

m3

In the marine species of the chelonian order, of which this may be regarded as the type, the ossification of the carapace and plastron is less extensive, and the whole skeleton is lighter, than in the boxtortoise (Fig. 20), or any of those species that live on dry land. The head is proportionally larger,-a character common to aquatic animals; and, being incapable of retraction within the carapace, ossification extends in the direetion of the fascia covering the temporal muscles, and forms a second bony covering of the cranial cavity: this accessory defence is not due to the intercalation of any new bones, but to exogenous growths from the frontals, 11, postfrontals, 12, parietals, 7, and mastoids, 8.

v2

42

43

441

v3

45

b

24

The carapace (Fig. 21) is composed of a series of median and symmetrical pieces ch, s1 to s11, and of two series of unsymmetrical pieces on each side. The median pieces have been regarded as lateral expansions of the summits of the neural spines; the medio-lateral

picces as similar developments of the ribs; and the marginal pieces as the homologues of the sternal ribs. But the development of the carapace shows that ossification begins independently in a fibro-cartilaginous matrix of the corium in the first, ch, and some of the last, s9 to s 11, median plates, and extends from the summits of the neural spines into only eight of the intervening plates, s 1 to 88: ossification also extends into the contiguous lateral plates, pl1 to pl 8, in some chelonia, not from the corresponding part of the subjacent ribs, but from points alternately nearer and farther from their heads, showing that such extension of ossification into the corium is not a development of the tubercle of the rib, as has been supposed. Ossification commences independently in the corium in all the marginal plates, m1 to py, which never coalesce with the bones uniting the sternum with the vertebral ribs, and which are often more numerous, and sometimes less numerous than those ribs, and in a few species are wanting. Whence it is to be inferred that the expanded bones of the carapace, which are supported and impressed by the thick epidermal scutes called "tortoise-shell," are dermal ossifications, homologous with those which support the nuchal and dorsal epidermal scutes in the crocodile. Most of the pieces of the carapace being directly continuous or connate

PLASTRON OF THE TURTLE.

215

with the obvious elements of the vertebræ, which have been supposed exclusively to form them by their unusual expansion, the median ones, s1 to s11, have been called "neural plates," and the medio-lateral pieces, pl 1 to pl 8, "costal plates;" but the external lateral pieces, m 1 to m 12, have retained the name of "marginal plates." The first or anterior of the median plates (ch, “nuchal plate”) is remarkable for its great breadth in the turtles, and usually sends down a ridge from the middle line of its under surface, which articulates more or less directly with the summit of the neural arch of the first dorsal vertebra; the second neural plate is much narrower, and is connate with the summit of the neural spine of the second dorsal vertebra: the seven succeeding neural plates have the same relations with the succeeding neural spines: the rest are independent dermal bones. The costal plates of the carapace are superadditions to eight pairs of the pleurapophyses or vertebral portions of the second to the ninth ribs inclusive. The slender or proper portions of these ribs project freely for some distance beyond the connate dermal portions, along the under surface of which the rib may be traced, of its ordinary breadth to near the head, which liberates itself from the costal plate to articulate to the interspace of the two contiguous vertebræ, to the posterior of which such rib properly belongs.

Fig. 22.

es

es

The plastron, or floor of the bony house, consists in the genus Chelone, as in the rest of the order, of nine pieces,-one median and symmetrical, and the rest in pairs. With regard to the homology of these bones, three explanations may be given: one in conformity with the structure of the thoracic-abdominal cage in the crocodile; the other based upon the analogy of that part in the bird; and the third agreeably with the phenomena of development. According to the first, the median piece of the plastron, called "ento-sternal," S, answers to the sternum of the crocodile, or 66 sternum proper," and the four pairs of plastron-pieces, es, hs, ps, xs, answer to the "homapophyses" forming the so-called sternal and abdominal ribs of the crocodile. Most comparative anatomists have, however, adopted the views of Geoffroy St. Hilaire, who was guided in his determination of the pieces of the plastron by the analogy of the skeleton of the bird; according to which all the parts of the plastron are referred to a complex and greatly developed sternum, and the marginal plates are viewed as sternal ribs (hæmapophyses). The third ground of determination refers the parts of the plastron, like those of the carapace, to a combination of parts of the endoskeleton with those of the exoskeleton.

хо

ps

PLASTRON OF CHELONE CAOUANNA.

In Fig. 21, the marginal plates, m1 to m 12, are twenty-four in number, or twentysix if the first (nuchal, ch) and last (pygal, py) vertebral plates be included. Omitting: these in the enumeration, three marginal picces intervene on each side at the angles between the first median plate and the point of the first costal plate formed by the end of

216

VERTEBRE OF THE TORTOISE.

the second dorsal rib, which point enters a depression in the fourth marginal piece, m 4; the fifth, sixth, seventh, eighth, ninth, and tenth marginal plates are similarly articulated by gomphosis to the six succeeding ribs; the eleventh marginal plate has no corresponding rib; the twelfth is articulated with the point of the ninth dorsal rib supporting the eighth costal plate.

Fig. 23.

ns

The want of concordance with the vertebral ribs, or "pleurapophyses," arising from the increased number of the marginal pieces, favours the idea of their being dermal ossifications, such peripheral elements being more subject to vegetative division and multiplication than the hæmapophyses: the absence of the marginal pieces in the trionyx gives additional support to the same view. The median piece, S, is here regarded as a hæmal spine: it is called "entosternum." The parial pieces of the plastron are the "hæmapophyses" connate with expanded dermal ossifications, and have received the following special names: es, "episternal;" "hs,

hs

SEGMENT OF CARAPACE AND PLASTRON.

"hyosternal;" ps, "hyposternal;" xs, "xiphisternal."

In some extinct chelonia the number of these lateral elements of the plastron is increased by an intercalated pair which I have called "mesosternals." In the figure of the segment, as modified to form the carapace and plastron (Cut 23), the nature of the bones is indicated by the letters according to the explanation given of the archetype vertebræ (Fig. 5, p. 169), the dermal superadditions being marked sc.

In the figure of the skeleton of the box-tortoise (Fig. 20) a section of the carapace and plastron has been removed from the right side to expose the dorsal and sacral vertebræ, and the disposition of the scapular and pelvic arches. The eight cervical vertebræ are free, moveable, and ribless; the fourth of these vertebræ has a much elongated centrum, which is convex at both ends; the eighth is short and broad, with the anterior surface of the body divided into two transversely elongated convexities, and the posterior part of the body forming a single convex surface divided into two lateral facets; the under part of the centrum is carinate. The neural arch, which is anchylosed to this centrum, is short, broad, obtuse, and overarched by the broad expanded nuchal plate, ch. The first dorsal vertebra, d 1, is also short and broad, with two short and thick pleurapophyses, articulated by one end to the expanded anterior part of the centrum, and united by suture at the other end to the succeeding pair of ribs. The head of each rib of the second pair is supported upon a strong trihedral neck, and articulated to the interspace of the first and second dorsal vertebræ : it is connate, at the part corresponding to the tubercle, with the first broad costal plate, which articulates by suture to the lateral margin of the first neural plate, and to portions of the nuchal and third neural plates: the connate rib, which is almost lost in the substance of the costal plate, is continued with it to the anterior and outer part of the carapace, where it resumes its subcylindrical form, and articulates with the second and third

marginal pieces of the carapace. The neural arch of the second dorsal vertebra is shifted forwards to the interspace between its own centrum and that of the first dorsal vertebra. A similar disposition of the neural arch and spine and of the ribs prevails in the third to the ninth dorsal vertebræ inclusive. The corresponding seven neural plates are connate with the spines of those vertebræ, and form the major part of the median pieces of the carapace; the corresponding costal plates, anchylosed to the ribs, form the medio-lateral pieces; the ninth, tenth, and pygal plates, with the marginal plates of the carapace, do not coalesce with any parts of the endo-skeleton. The bony floor of the great abdominal box, or "plastron," is formed by the hæmapophyses and sternum connate with dermal osscous plates, forming, as in the turtle, nine pieces, one median and symmetrical, answering to the proper sternum, and eight in pairs: but they are more ossified, and the hyo- and hypo-sternals unite suturally with the fourth, fifth, and sixth marginal plates, forming the side-walls of the bony chamber. The junction between the hyo- and hypo-sternals admits of some yielding movement. The iliac bones, 62, abut against the pleurapophyses of the tenth, eleventh, and twelfth vertebræ, counting from the first dorsal vertebra. These three vertebræ form the sacrum: their pleurapophyses are unanchylosed, converge, and unite at their distal extremities to form the articular surface for the ilium. Beyond these the vertebræ, thirty-five in number, are free, with short, straight, and thick pleurapophyses, articulated to the sides of the anterior expanded portions of the centrums. They diminish to mere tubercles in the first caudal vertebra, and disappear in the remainder. The neural arches of the caudal vertebræ are flat above, and without spines. The strong columnar scapula, 51, is attached by ligament to the first costal plate, and, retaining its primitive rib-like form, it descends almost vertically to the shoulder-joint, of which it forms, in common with the coracoid, 52, the glenoid cavity. A strong subcylindrical process or continuation of the scapula, representing the acromion, bends inwards to meet its fellow at the middle line. The coracoid continues distinct from the scapula, expands, and becomes flattened at its median extremity, which does not meet its fellow or articulate with the sternum. The iliac bones, 62, are vertical and columnar, like the scapula, but are shorter and more compressed: they articulate, but do not coalesce, with the pubis, 64, and ischium, 63. The acetabulum is formed by contiguous parts of all the three bones. The pubis arches inwards, and expands to join its fellow at the median symphisis and the ischium posteriorly. It sends outwards and downwards a long thick obtuse process from its anterior margin. The ischia, in like manner, expand where they unite together to prolong the symphysis backwards. In the skull the parietal crista is continued into the occipital one without being extended over the temporal fossæ, as in the turtle; the fascia covering the muscular masses in these fosse undergoing no ossification. The bony hoop for the membrana tympani is incomplete behind, and the columelliform stapes passes through a notch instead of a foramen to attain the tympanic membrane. The mastoid is excavated to form a tympanic air-cell. In the Australian long-necked terrapene (hydraspis longicollis) the head is much depressed, the mastoids are excavated by large tympanic cells, and prolonged backwards: the frontal is produced forwards as far as the anterior nostril, where it terminates in a point between the two nasals, which are here distinct from the prefrontals. The margins of the upper and lower jaws are trenchant: the hypapophysis of the atlas has the form of a diminutive wedge-bone, forming as usual the lower part of the articular cup for the occipital condyle: the rest of the body of the atlas, or 66 odontoid," " has coalesced with its proper neural arch, which developes two transverse and two long posterior oblique processes, as in the chelys.