File:Image from page 233 of "Water reptiles of the past and present" (1914) (14586390660).jpg

Identifier: waterreptilesofp1914will Title: Water reptiles of the past and present Year: 1914 (1910s) Authors: Williston, Samuel Wendell, 1851-1918 Subjects: Aquatic reptiles Publisher: Chicago, Ill., The University of Chicago Press Contributing Library: Boston Public Library Digitizing Sponsor: Boston Public Library

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Text Appearing Before Image: e numbers of phalanges—that is, the bones of thefree digits—are like those of mammals, that is, two in the firstand three in each of the other four digits. The land tortoises havelost some of these, while the river turtles have either gained oneor two in the fourth finger and fourth toe, or else have enjoyedan uninterrupted descent from the primitive reptiles which normally 222 WATER REPTILES OF THE PAST AND PRESENT possessed that number. All other reptiles, save those phylogeneti-cally allied to the primitive mammals, that is, the Theriodontiaand their allies, have normally the phalangeal formula 2,3,4, 5, 4.It was partly because of this similarity of the numbers of toe bonesthat the turtles have been classed in the great group of reptilesthat includes the ancestors of the mammals; that is, under thistheory, the turtles would enjoy a nearer relationship to the mam-mals and to man himself than any other living reptiles! But thisclassification has been shown to be quite artificial.

Text Appearing After Image: 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 Cheloniarepresent in themselves one of the primary subdivisions of the classReptilia, and that, unlike most others, the order has enjoyed a mostremarkable longevity. And doubtless they are one of the primarybranches of the reptilian stock, which has remained distinct sincePermian times at least, if not since Carboniferous, isolated andremarkably homogeneous, giving off no branches which departedfar from the main stock, and on the whole leading a singularlyplacid existence for ten or more million years. In most textbooks the order Chelonia is divided into threesuborders, the Pleurodira, the Cryptodira, and the Trionychoidea. CHELONIA 223 In recent years, however, the earlier members of the older groupof Pleurodira

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