File:Image from page 99 of "Water reptiles of the past and present" (1914) (14773006625).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: 88 WATER REPTILES OF THE PAST AND PRESENT immovable. The lower jaws, always rather slender, are firmlyunited in front, sometimes for a long distance, as in the moderngavials. The teeth of the broad-headed plesiosaurs are long,slender, pointed, and recurved, of a murderously cruel shape; theyare deeply implanted in sockets, and number from twenty tothirty on -each jaw above and below. There are no teeth on thebones of the palate, such as the mosasaurs possessed. The slender-jawed, gavial-like plesiosaurs have more numerous, but smallerteeth. The surface of the skull on each side behind, for the attach-ment of the muscles closing the mandibles, is of great extent; insome this surface is increased by a high, thin crest in the middle,as in strongly carnivorous animals, all of which give conclusiveevidence of the powerful muscles used in biting and seizing. Thereis but one temporal opening on each side, as in the ichthyosaurs

Text Appearing After Image: Fig. 41.—Skull of Trinacromerum from the side: ang, angular; d, dentary; pm,premaxilla; po, postorbital; j, jugal; sur, surangular. and the mosasaurs, whereas the crocodiles, thalattosaurs, phyto-saurs, etc., have two. The brain cavity of all plesiosaurs is small,though the cavities of the internal ears, the semicircular canals atleast, are large. The semicircular canals in vertebrates have littleor nothing to do with the function of hearing; they serve rather forequilibration, for the co-ordination of muscular movement; possi-bly we may infer from their large size in the plesiosaurs that theywere not at all clumsy in their movements. There is a large open-ing for the pineal body, the so-called eye in the roof of the braincavity, though its possession does not necessarily imply the pos-session of a functional organ. The Plesiosauria included some of the largest aquatic reptilesthat have ever existed, equaled, perhaps, though not exceeded,by some of the extinct crocodiles. The larges

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