Page:The Osteology of the Reptiles.pdf/240

222

Temporal region of skull usually exposed by the emargination of the roof bones, the supratemporals, dermosupraoccipitals, and tabulars absent. Eight cervicals. Ten dorsal vertebrae enclosed in a more or less complete carapace; an ossified plastron. A single coracoid; pelvis with large openings. Phalangeal formula always reduced.

No order of reptiles is so unequivocally distinguished from all others as the Chelonia, the turtles and tortoises. Jaws are always [beaked], and except in Stegochelys of the Trias they are wholly toothless; a short, broad body, a rather short skull, a flexible neck of eight vertebrae, and an osseous carapace and plastron.

In addition to the bones mentioned above, the postfrontals and ectopterygoids and usually the lacrimals are absent, the temporal region, when roofed over, covered by the large postorbitals, jugals, squamosals, and quadratojugals. The prevomers are united; there is no interpterygoidal opening, and there may be an incipient secondary roof to the palate. The prefrontals meet in the median line, the nasals are often absent; the stapes is slender. The pectoral girdle is composed of a furcate scapula and a single coracoid, usually without a supracoracoid foramen. The carpus and tarsus are much modified, and the phalangeal formula is always reduced, to 2, 3, 3, 3, 3 or 2, 2, 2, 2, 2, with the fifth toe usually still more so. Because of the inflexible carapace the structure and posture of the limbs are much modified. The forearm is so twisted upon the humerus that the foot is brought more or less directly forward in walking. As in the dinosaurs there is a greater or less reduction of the postaxial fingers and a strengthening of those on the other side of the feet. Only rarely, in certain aquatic types, have the outer fingers of the hand more than three phalanges, probably because of an incipient hyperphalangy.

Regarding the general classification of the Chelonia there is still dispute. According to Cope, Dollo, and Hay there are two chief divisions or suborders, the Athecae and Thecophora, dependent upon the character of the carapace, the former with but a single living species, the latter with more than two hundred. The more generally accepted classification recognizes four suborders, the Cryptodira (