File:Image from page 89 of "Water reptiles of the past and present" (1914) (14769824211).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: theneck among mammals is always due to an increase in the length ofthe individual bones, never to an increase in the number from seven,with but a single exception—a South American sloth which has 78 WATER REPTILES OF THE PAST AND PRESENT nine cervical vertebrae. The long neck of birds is due both to anincrease in the length of the individual vertebrae and to an increasein their number, to as many as twenty-one. But the elongation ofthe neck among plesiosaurs was very variable indeed; sometimesit was ten or twelve times the length of the head, at other times itwas even shorter than the head. And the number of bones com-posing it was also extremely variable, scarcely any two specieshaving the same, the known extremes being seventy-six andthirteen. In Elasmosaurus platyurus, for instance, the longest-necked plesiosaur known, the head was two feet in length, theneck twenty-three, the body nine, and the tail about seven; on theother hand, in the shortest-necked plesiosaur known, Brachau-

Text Appearing After Image: FiG. 32.—Skeleton of Trinacromerum osbomi, a Cretaceous plesiosaur, as mountedin the University of Kansas Museum. chenius Lucasi, the head was two and one-half feet in length, theneck less than two feet, and the body about five; the length of thetail is unknown. Not only was the number of vertebrae so extraordinarilyincreased in many plesiosaurs, but in the longest necks the verte-brae themselves, as in birds, were more or less elongated, especiallythe posterior ones, which may be six or seven times the length ofthe anterior ones. Not only was the neck of such great lengthin many plesiosaurs, but it also tapered very much toward thehead. The vertebrae are always biconcave, but the cavities are shallow,saucer-like, sometimes almost flat at each end, and very differentfrom the conical fish-like cavities of ichthyosaurian vertebrae. SA UROPTERYGIA

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