The sea-turtles, or Chelonidae comprise five or six living species, inhabitants for the most part of tropical and subtropical oceans, of which the green or edible turtle (_Chelone_), the hawksbill turtle (_Caretta_), and the loggerhead (_Eretmochelys_ [Fig. 119]) are the best known. They are all thoroughly aquatic in habit, and of large size, from three to five feet in length. The carapace is heart-shaped, and reduced, that is, with large openings between the ribs; the plastron also is reduced and loosely united to the carapace. The neck is short and the head is not retractile within the shell. The temporal region of the skull is roofed over. The four legs form large and powerful flippers, and the hind legs are relatively small. The body is flattened and the tail is small. The aquatic characters of the limbs are seen especially in the broad and strong humerus, with the radial crest for the attachment of powerful muscles situated far down on the shaft; in the relative shortness of the radius and ulna, and the large size of the latter bone; in the flattened carpal bones; and in the great elongation of the digits and the absence of all but one or two of the claws. Unlike the leather-back turtle and the Cretaceous sea-turtles, the carapace and plastron are completely covered with horny shields, from which indeed the tortoise shell of commerce is derived. Except the green turtle, all members of the family are carnivorous. [Illustration: FIG. 119.—_Eretmochelys_, loggerhead turtle. (By permission of the New York Zoölogical Society.)] Extinct members of the family are known from scanty remains in Cenozoic and late Cretaceous rocks. From the earlier Cretaceous deposits of the plains more primitive allied forms occur, often classed in distinct families of which _Toxochelys_ (Fig. 120) and _Desmatochelys_ are the more noteworthy. The latter genus, especially, might well have been an ancestor of all the modern forms. About three feet in length, it had all the essential characteristics of the sea-turtles, in its thin form, roofed-over skull, reduced carapace, loose plastron, and flipper-like limbs. The single known specimen, preserved in the museum of the University of Kansas, came from the lower rocks of the Upper Cretaceous of Nebraska. Yet earlier, at the close of the Jurassic, there were shore turtles of considerable size that had begun to develop a fondness for the open seas; to acquire a depressed form and lightened shell, the limbs still retaining, however, more of the terrestrial or crawling form. They are grouped as a separate family, the Thalassemydae, and include the first of the Chelonia to depart from the marsh and fresh-water habits which for long ages, perhaps, had limited the activities and evolution of the turtles. [Illustration: FIG. 120.—Carapace of _Toxochelys bauri_, an Upper Cretaceous sea-turtle: _ep_, epineural. (After Wieland.)] ANCIENT SEA-TURTLES. PROTOSTEGIDAE