SAURO PTERYGIA 91 



At the same time this union was ligamentous only, not bony and 

 unyielding, since the limbs were never used to support the body 

 upon the ground; and it is of interest to observe that the ilia are 

 directed, not upward and forward, but upward and backward to 

 the sternum, precisely the position that would be expected with the 

 force or thrust coming from behind, and not below the yielding 

 ligaments. Were the tail longer and more powerful, the hind limbs 

 would have been smaller and weaker, of use chiefly in equilibration, 

 involving the loss of any connection with the vertebral column and 

 the disappearance of the sacrum. It is of interest, finally, to 

 observe that many of the slender- jawed plesiosaurs had a relatively 

 short neck; they were doubtless more distinctively fish-eating in 

 habit, and possessed greater speed. That the limbs of plesiosaurs 

 were powerful propelling organs is also conclusively proved by their 

 structure. Quite unlike all those animals whose locomotion in the 

 water is chiefly effected by the tail, the humeri and femora, the 

 upper arm and thigh bones were elongated, and not shortened. 

 They form the rigid and stout handles of oars whose blades are the 

 thinner, flexible forearm, wrist, and fingers, or the corresponding 

 foreleg, ankle, and toes. No other purely aquatic reptiles, save 

 the turtles, which likewise are of the oar-propelled type, have 

 elongated arm and thigh bones. 



Textbook illustrations of the plesiosaurs usually depict the 

 necks, like those of the swans, freely curved, and a popular scientific 

 article in one of our chief magazines a few years ago depicted one of 

 them with the neck coiled like the body of a snake. One noted 

 paleontologist, indeed, not many years ago described the plesiosaurs 

 as resting on the bottom in shallow waters with the neck uplifted 

 above the surface viewing the waterscape! And when we con- 

 sider the fact that some species of the elasmosaurs had a neck not 

 less than twenty feet in length, such a flexible use of it would not 

 seem improbable. But the plesiosaurs did not and could not use 

 the neck in such ways. They swam with the neck and head, how- 

 ever long, directed in front, and freedom of movement was restricted 

 almost wholly to the anterior part. The posterior part of the neck 

 was thick and heavy, and could not have been moved upward or 

 downward to any considerable extent and not very much laterally. 



