File:Image from page 253 of "Water reptiles of the past and present" (1914) (14770758774).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: rful, flattenedpaddles, not unlike those of Eretmochelys, but wholly destitute ofclaws. The front paddles are much larger than the hind ones;the humerus is long and flattened, and the digits are elongated.The leather-back is a powerful and effective swimmer, going longdistances. Its habits are not well known; its food is chiefly fish,crustaceans, and mollusks. So very different is the structure of its shell that some excellentnaturalists regard Dermochelys as the equivalent in rank of all otherturtles combined, the sole representative of the suborder Athecae, 242 WATER REPTILES OF THE PAST AND PRESENT as distinguished from the Thecophora. Dr. Hay, whose authorityon fossil turtles is of the highest, believes that its line of ancestry hasbeen distinct from that of all other turtles from Triassic times atleast. Others believe that the leather-back is merely a highlyspecialized form derived from the ordinary shelled type, a de-scendant of some of the marine turtles of Cretaceous times. In

Text Appearing After Image: Fig. 128.—Dermochelys coriacea. (From Brehm) support of the primitive ancestry of the leather-back Dr. Hayoffers the following: The writer holds the view that the earliest turtles possessedpractically two kinds of shell, one purely dermal, consisting prob-ably of a mosaic of small bones arranged in at least twelve longi-tudinal zones. Each zone probably consisted of a row of largerbones bordered on each side by smaller bones. Each of these boneswas covered by a horny scute. The nearest approach to such adermal shell is in our days seen in Dermochelys. Beneath the skinthere seems to have existed a carapace more or less complete, whichconsisted of a nuchal, a median row of neurals, eight pairs of costals, CHELONIA 243 a pygal, probably one or more supraneurals, and about elevenperipherals on each side. To what extent the neurals and thecostal plates had become anchylosed to the neural spines and theribs respectively, it is now impossible to determine. Nor can wesay to what extent the

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