File:Image from page 53 of "Water reptiles of the past and present" (1914) (14749974936).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: onsistedof nine bones, two in thefirst row, two in the second, and five inthe third, but in all modern reptiles thebones of the middle row and the fifth onein the third row have disappeared; in somelizards and turtles the two of the first roware fused. The two bones of the proximalrow correspond quite to the astragalus andcalcaneum, the astragalus articulating withboth tibia and fibula proximally, the calcaneum with the latteronly. The eldest known tarsus of any vertebrated animal, onefrom the Coal Measures of Ohio, has this structure, while in allthe early amphibians there were three bones, the tibiale, inter-medium, and fibulare. Some of the later swimming reptiles, likethe ichthyosaurs and plesiosaurs, have apparently this amphibianstructure, with three bones that are usually called tibiale, inter-medium, and fibulare, but it is very doubtful indeed whetherthey are homologically the same. In the middle row two centraliaare known in one or two very ancient reptiles, but for the most

Text Appearing After Image: Fig. 23.—Right hindfoot of Ophiacodon: a,astragalus; c, calcaneum;ex, C2, centralia; 1, 2, 3,4, 5, tarsalia. THE SKELETON OF REPTILES 43 part there is only a single centrale, and even that is usuallylost in later reptiles. The third row, like the third row of thecarpus, had a distinct bone for each digit originally, but the fifthone was very soon lost and has never reappeared. The structureof the digits and number of bones are quite like those of the hands,except that the fifth toe has four bones instead of three, that is,the phalangeal formula was 2, 3, 4, 5, 4. As a rule in terrestrialreptiles, as in terrestrial mammals, the hind foot is more specializedthan the front ones. Most reptiles have an external covering or exoskeleton of hornyplates or scales or bony scutes. Horny scales are of course notpreservable as petrifactions, though in many instances theiractual carbonized remains or their impressions have been detected.Such information comes only rarely, though doubtless in thec

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