WILLISTON : AQUATIC AIR-BREATHING VERTEBRATES. 265 



body, and its loss was to be expected In Geosaurus, with its 

 ambulatory hind limbs, the sacrum has not disappeared. The 

 ilium, where present, is small, merely a connecting rod, united 

 by ligament to some transverse process. In the mosasaurs it is 

 directed forward from the acetabulum ; in the plesiosaurs, on 

 the contrary, its position is upward and backward. 



Why the pectoral and pelvic girdles should have been so re- 

 markably developed on the under side of the body in the 

 plesiosaurs is not clear, forming, as they do, a nearly complete, 

 convex, bony armor on the under side of the trunk, supple- 

 mented by the abdominal ribs. In the ichthyosaurs, with the 

 less broad expansion of their girdles, the more numerous ventral 

 ribs may have served a similar end. Abdominal ribs are found 

 in the crocodiles, but there are none in the mosasaurs. 



It is a remarkable fact that in all aquatic vertebrates, except 

 the bottom-feeding sirenia, and the ichthyosaurs, of which I can 

 get no information, the bones of the skeleton have become more 

 cancellated and spongy. The oily character of the pinniped 

 bones is well known, as also the great sponginess of the ceta- 

 cean bones. Among the plesiosaurs a similar soft and friable 

 structure of the bones is painfully evident, and in the mosasaurs 

 there is a very distinctly greater softness and sponginess of the 

 bones in the more specialized types. 



Nakedness of the skin is of course to be expected among 

 swimming animals ; hair has almost disappeared in the mam- 

 mals, and scales or plates in the reptiles ; in the mosasaurs 

 alone scales are still persistent, but they are relatively small 

 and smooth. 



It is very evident that the plesiosaurs were not cpuick-swim- 

 ming animals ; the body was relatively broad, flat, and thick- 

 set, very unfish-like in form ; the paddles were small as compared 

 with the size of the body, and the tail was not of sufficient ca- 

 pacity to compensate for their incapacity. The more or less 

 elongated neck — for in none was the neck short in the sense of 

 the ichthyosaurs — did not give any advantages in movement. 

 The movements of these animals must have been more like those 

 of the turtles than of the mosasaurs or crocodiles. The head 

 was relatively small, and the narrow gullet, between unyield- 

 ing bones, permitted the deglutition of small animals only — very 

 small indeed as compared with the size of the body. The 

 spongy structure of the bones clearly indicates that they were 



