File:Image from page 164 of "Water reptiles of the past and present" (1914) (14750036576).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: trongteeth implanted in the back part of thepalate, upon bones called pterygoids, the useof which will be understood later. The teethwere inserted on large, tumid, bony bases,rather loosely attached in shallow pits oralveoli, unlike the teeth of all modern lizards.Such a mode of attachment of the teethdoubtless had some relation to the habits ofthe animals concerning which we are not quite clear. They were easily dislodged, and, in consequence, ofvery unequal size, some full grown, some small, and others justappearing above the surface of the gums in the living animals.The frequent loss of teeth and their constant and easy replace-ment by new ones is a peculiarity of predaceous reptiles, therebyinsuring their best functional use. The external nostrils, of large size, were situated at a consider-able distance back of the end of the snout, but not nearly so farback or so near the eyes as were the nostrils of the ichthyosaurs,plesiosaurs, and phytosaurs. Their size and position suggest a

Text Appearing After Image: Fig. 70.—Tooth ofTylosaurus, two-thirdsnatural size. i54 WATER REPTILES OF THE PAST AND PRESENT use like that of the modern aquatic monitors, as mentioned on apreceding page. The eyes were of moderate size, those of the lesspurely aquatic forms being directed more laterally than those ofspecies of more distinctly diving habits. They were protectedby a stout ring of bony plates, as were the eyes of all truly aquaticreptiles of the past. The ears, also, in most if not all mosasaurs,had a thick cartilaginous ear-drum in place of a simple membrane,evidently, as Dollo has shown, for better protection under unduepressure of the water in deep diving. As in all other lizards, the bones with which the lower jawsarticulate, the quadrates, were loosely attached at the upper end,permitting great freedom of movement in all directions, more eventhan the land lizards have. The lower jaws were long and powerful,armed with a- single row of teeth on each side, from sixteen to

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