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 _Ornithorhynchus_ 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 in those reptiles allied to the ancestors of the mammals has the scapula ever had a spine or projection on its dorsal side. [Illustration: FIG. 18.—_Cacops_, a Permian stegocephalian, ancestrally allied to the primitive reptiles, with rhachitomous vertebrae and large cleithrum above the scapula.]