NORTH AMERICAN PLESIOSAURS 



723 



two vascular foramina, but they are small, and the ridge between them 

 is obsolete. Posteriorly the centra become more nearly circular in 

 outline, and the articular surfaces are nearly jfiat, the under border also 

 nearly straight. The transverse processes here are more slender and 

 are directed more obliquely upward. The zygapophyses are smaller 

 and weaker. 



Lengths 63, 63, 66, 70, 72, 72, 72, 72 



Widths 85, 85, 97, 97, 97, 97, 97, 97 



Heights of centra 86, 86, ? 97, 97, 97 



Sacrum. — The structure of the sacrum is so well shown in the 

 figures (figs. 7, 8) that a detailed description is unnecessary. The 

 specimen is of especial _^ 



interest as for the first 

 time giving a complete 

 knowledge of this part 

 of the vertebral column 



in the plesiosaurs. Three ^^ 



vertebrae, it is seen, take 

 part in the structure of 

 the sacrum. A fourth in 

 front, probably, as in 

 Pantosaurus, has its sa- 

 cral rib arising from the 



centrum in part and par- Fig. 8.— T. bentonianum. Sacrum from right 



........ , side, one-fourth natural size. 



ticipatmg m the support 



of the ilium, but if so it was not preserved with the remainder of this 

 skeleton. The two posterior vertebrae are co-ossified, the front one 

 not. The anterior sacral rib was connected with the ilium by Hga- 

 mentous union, a rugosity for which is usually observed at a Httle 

 distance from the extremity of the ilium. 



The opinion has been expressed at various times that the plesio- 

 saurs are related to the turtles because of the position of the ifia, 

 directed as they are downward and forward. But I see no necessity 

 for such an explanation of the resemblances here. The hind hmbs 

 in the turtles, as in the plesiosaurs, are used chiefly as propelling 

 organs. The strain upon the ihum at the acetabulum would be 

 antero-posterior, inclining the iHum forward. While the union with 



