File:Image from page 48 of "Water reptiles of the past and present" (1914) (14586272590).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: Fig. 19.—Scapula (sc), coracoid (cor), and metacoracoid (mcor) of Dimeirodon The cleithrum is known in only a few of the old reptiles; itis a more or less slender bone which lies along the upper frontmargin of the scapula, articulating at its lower end with the upperend of the clavicle on each side. THE SKELETON OF REPTILES 37 The breast bone or sternum, while not properly a part of thepectoral girdle, may be mentioned here. In reptiles it is rarelywell developed or even ossified, the flying reptiles known as thepterodactyls being the most notable exceptions. It was a com-paratively late development in this class, the earliest ones notpossessing it even in a cartilaginous condition. It was doubtlessevolved from the more or less numerous and slender ossifications onthe under side of the body called ventral or abdominal ribs, after

Text Appearing After Image: Fig. 20.—Clavicles and interclavicle of Ophiacodon, a theromorph reptile fromthe Permocarboniferous of Mew Mexico. the coracoids had become reduced and more slender. Wheneverit is present the coracoid articulates with it on each side in front.In most lizards it remains as a cartilage throughout life. ANTERIOR EXTREMITY The upper arm bone, or humerus, like most other bones of theextremities, has been greatly modified by the habits of the differentreptiles. In running and climbing reptiles it is always slender,while in burrowing reptiles it is short and stout and much expandedat the extremities, like the humerus of the mole among mammals.And we shall also see how greatly modified it was among the 38 WATER REPTILES OF THE PAST AND PRESENT swimming reptiles. The humerus of flying reptiles has an enor-mous process on the side, corresponding to the attachment of thedeltoid muscle. The head of the humerus, for articulation with theglenoid cavity of the scapula, is rounded in all reptiles,

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