I40 THE OSTEOLOGY OF THE REPTH^ES 



short and small, of little service for muscular attachment, unlike the 

 scapulae of tail-propelling aquatic reptiles. 



Probably the great development of the ventral elements of the 

 pectoral and pelvic girdles in the plesiosaurs implies greatest de- 

 velopment of the ventral muscles, used in the antero-posterior and 

 downward movement of the paddles. A clavicular process of the 

 coracoids of the later plesiosaurs (Fig. 102) extends forward to articu- 

 late with the proscapular process or with the clavicles. The mode of 

 development of the proscapular process, as shown by Andrews, 

 proves that it is an exogenous process of the scapula, corresponding 

 to the acromion and not to the procoracoid, as it was once thought 

 to do. The scapulae of tail-propelling aquatic reptiles are always 

 short and broad, fan-shaped (Figs. 85, 112). The scapula of the 

 Chelonia is also peculiar (Fig. 109 b, c, d). Enclosed within the 

 thoracic cavity it has two rather slender branches, one extending 

 toward the roof; the other, the proscapular process, springing from 

 near the articular fossa, is directed downward and inward to be 

 attached by ligaments to the interclavicle or entoplastron. Formerly 

 this process was also supposed to be a separate ossification, the pro- 

 coracoid, fused with the scapula, and on the strength of it a rela- 

 tionship was found with the plesiosaurs. It is now known to be an 

 exogenous process of the scapula. The coracoid is more or less 

 flattened and dilated at its extremity. It is directed inward and 

 backward, and is connected with its mate by ligaments. In Stego- 

 chelys, a Triassic turtle, the proscapular process is small (Fig. 100). 



In Eunotosaurus . a Permian genus of South Africa, that has been 

 referred to the Chelonia in a wide sense, the pectoral girdle is of the 

 primitive type, having a moderately long scapula, slender clavicles, 

 and interclavicle, and the two coracoids approximating their mates 

 in the median line. 



A distinctly differentiated acromion process occurs in reptiles only 

 among the Pariasauridae and especially the therapsids, mammal-like 

 forms from South Africa. A distinct angular process on the front 

 margin of the scapula in the Cotylosauria (Fig. 96 b, c) and Thero- 

 morpha (Figs. 96 d, 106), to which the clavicle is attached, however, 

 corresponds to the acromion. 



In general, the shorter and stouter are the legs, the shorter and 

 broader are the scapulae. In upright-walking reptiles the scapula is 



