File:Image from page 44 of "Water reptiles of the past and present" (1914) (14769807061).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: lly migrated up onthe arch and on the transverseprocess (diapophysis), so thatboth head and tubercle areattached to the diapophysis;and this condition is equallycharacteristic of the orders ofreptiles known collectively asthe Archosauria—the crocodiles, pterodactyls, dinosaurs, and phytosaurs. In the Sauropterygia, theribs are single-headed and attached to the end of the diapophy-sis. Finally in most ichthyosaurs the capitulum and tubercleboth articulate with the body of the vertebra. Ribs primitively were probably attached to all the vertebraeto the end of the tail. In the earliest reptiles that we know theyare present on all vertebrae as far back as the tenth or twelfthcaudal only, those of the caudal for the most part co-ossifiedwith the centra. The ribs of the neck vertebrae more quicklydisappeared, or became fused with the vertebrae, and only in thecrocodiles among living reptiles are there ribs on the atlas. Thesacral ribs, on the other hand, became much larger and stouter and

Text Appearing After Image: Fig. 17.—Ostodolepis, a primitivetheromorph reptile. Vertebrae from infront and side, with primitive double-headed rib and intercentrum. 34 WATER REPTILES OF THE PAST AND PRESENT developed an articulation at their outer ends for the support of theilium (Fig. 16). The so-called ventral ribs are slender ossifications in the con-nective tissue under the skin, on the under side of the body, andare characteristic of most reptiles. The anterior ones doubt-less fused together more or less to form the sternum or breastbone, which was otherwise absent in the early reptiles. PECTORAL OR SHOULDER GIRDLE Those bones which form the framework for the support of theanterior extremity in vertebrate animals are known collectivelyas the pectoral girdle. In our own skeleton there are but two oneach side, or four in all, the scapula or shoulder-blade, and theclavicle or collar-bone. A third bone, however, is representedin all mammals by a mere vestige which early unites with thescapula and is called th

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