82 WATER REPTILES OF THE PAST AND PRESENT 



join 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 inter- 

 clavicle 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 under- 

 stood 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, 



