[Illustration: FIG. 117.—Limbs of _Colpochelys_, a recent sea-turtle: _H_, humerus; _R_, radius; _U_, ulna; _r_, radiale; _i_, intermedium; _u_, ulnare; _p_, pisiform; _c_, centrale; _T_, tibia; _F_, fibula; _a_, astragalus; _m_, fifth metatarsal. (From Wieland.)] From what has been said, it will be surmised that the Chelonia represent in themselves one of the primary subdivisions of the class Reptilia, and that, unlike most others, the order has enjoyed a most remarkable longevity. And doubtless they are one of the primary branches of the reptilian stock, which has remained distinct since Permian times at least, if not since Carboniferous, isolated and remarkably homogeneous, giving off no branches which departed far from the main stock, and on the whole leading a singularly placid existence for ten or more million years. In most textbooks the order Chelonia is divided into three suborders, the Pleurodira, the Cryptodira, and the Trionychoidea. In recent years, however, the earlier members of the older group of Pleurodira have been separated into a fourth suborder, the Amphichelydia, a group characterized by some not very important differences in the plastron and skull, and including those forms in which the cervical vertebrae are amphicoelous. This group continued to Eocene times before it became extinct, and consisted of archaic forms which persisted after all the other suborders had come into existence. The Cryptodira, especially characterized by the manner in which they withdraw the head and neck within the shell by an =S=-like vertical flexure, are known from the Lower Jurassic and are still the dominant group of today, with more than one hundred and forty living species. The Pleurodira in the narrower sense are first known from their remains in the Upper Cretaceous of North America and are still represented by about forty species, living in the Southern Hemisphere. They are distinguished from the other groups by the manner in which they withdraw the neck and head into the shell, by a horizontal, sidewise flexure. The third suborder, the Trionychoidea, also began in Cretaceous times, so far as we know, and are represented by about seventy living species, chiefly in the Northern Hemisphere. They are especially characterized by the absence of bony marginal plates and the soft epidermis.