The shoulder-girdle or pectoral arch is strangely unlike that of any other reptiles. There is no breast bone, since the breast bone is a comparatively late development in reptiles, not appearing, probably, until after the plesiosaurs had begun their existence. Taking the place of the sternum, the very large and broad coracoidsjoin each other in the middle, forming a sort of subdermal armor on the under side of the body in front. In some of the largest plesiosaurs these two bones measured together about six feet in length by four in width. Though so very large they are thick only in front between the articulations of the forelegs. The shoulder-blades are much reduced in size and are extraordinarily modified. The blade proper, that is, that part extending backward and upward, is narrow and small, affording but little surface for the attachment of muscles. On the inner side, extending toward the middle in front of the coracoids, there is another projection, often broad and large, to which was attached the clavicles when present, and often this projection met its mate of the opposite scapula in the middle in front of the coracoids in a broad union. The clavicles or collar-bones are small and thin, and sometimes absent; they also are united in the middle posteriorly with the coracoids when the scapula did not intervene. And the interclavicle also is sometimes wanting. Altogether the pectoral bones form a very large, broad, and concave trough inclosing the whole of the under side of the anterior part of the body. This extensive surface must have furnished attachment to stout and strong muscles controlling the downward and inward motion of the paddles. There is a well-developed sacrum of three vertebrae for the support of the pelvis or hip bones. The reason for its persistence in animals so thoroughly adapted for life in the water will be understood later. The ilium is slender; it was attached to the sides of the sacrum by ligaments, only, not forming a firm union, but strong nevertheless. The pubes and ischia, the other bones of the pelvis on the under side of the body, like the corresponding bones of the pectoral girdle, were enormously enlarged, forming great flat, bony plates. Besides these large bony plates of the shoulder and pelvic girdles, the short abdominal region was inclosed by numerous series of strong ventral ribs, that is, overlapping rod-like bones on each side, connected with a central piece. It will be seen that the whole under side of the body, from the base of the neck to the base of the tail, was well protected by bones, rigid and unyielding in front and behind, flexible for a short space below the abdomen; this surface, however, was not flat like the under shell of a turtle, but rounded from side to side. [Illustration: FIG. 36.—Pelvic girdle from above of _Trinacromerum osborni_, an Upper Cretaceous plesiosaur: _p_, pubis; _is_, ischium; _il_, ilium.]