34 WATER REPTILES OF THE PAST AND PRESENT 



developed an articulation at their outer ends for the support of the 

 ilium (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, and 

 are characteristic of most reptiles. The anterior ones doubt- 

 less fused together more or less to form the sternum or breast 

 bone, which was otherwise absent in the early reptiles. 



PECTORAL OR SHOULDER GIRDLE 



Those bones which form the framework for the support of the 

 anterior extremity in vertebrate animals are known collectively 

 as the pectoral girdle. In our own skeleton there are but two on 

 each side, or four in all, the scapula or shoulder-blade, and the 

 clavicle or collar-bone. A third bone, however, is represented 

 in all mammals by a mere vestige which early unites with the 

 scapula and is called the coracoid process. In the lowest forms 

 of mammals, the Monotremata, of which the Ornillwrhynchus and 

 Echidna are the only examples, not only is this coracoid bone 

 largely developed, articulating with the sternum or breast bone, 

 but there is an additional coracoid bone in front of this; and 

 there is also an interclavicle. Indeed, the pectoral girdle in 

 these mammals is more primitive or generalized in structure 

 than it is in any living reptiles, composed of scapula, coracoid, 

 metacoracoid, and clavicle on each side and an interclavicle 

 in the middle. No living reptiles have the metacoracoid, and, 

 as is the case with many mammals, some reptiles have no 

 clavicles. 



Primitively, that is, in all the old reptiles, the girdle is composed 

 of scapula, coracoid, metacoracoid, clavicles, and interclavicle, 

 while in some of the very oldest there is yet another bone, more or 

 less of a vestige, derived from the ancestral amphibians and called 

 the cleithrum or supraclavicle. The scapula is more or less 

 elongated in crawling and climbing reptiles; more slender and 

 bird-like in those which walked erect after the manner of birds and 

 mammals; shorter and more fan-shaped in the swimming reptiles, 

 as we shall see. In some pterodactyls, unlike all other known 

 animals, the scapula articulated at its upper end with the backbone, 

 giving a much firmer support for the anterior extremities. Only 



