142 University of Kansas Geological Survey. 



backward. Towards the end of the tail, the length of the cen- 

 tra decreases more rapidly than in the anterior parts, finally 

 terminating in a mere nodule of bone. In the well-preserved 

 specimen described there are seventy-seven vertebrae with chev- 

 rons, all continuous, except in one place. The last one is less 

 than a fourth of an inch in diameter, and shows that there had 

 been yet another, possibly several more. The entire series was 

 not less than seventy-eight and probably not more than eighty. 

 The chevrons are strongly oblique throughout, and are firmly 

 coossified with the centra. They are much more slender and 

 longer than in the other genera. 



The tail of Clidastes, as is thus seen, has a broad, vertical, 

 fin-like extremity, and doubtless aided very materially in the 

 propulsion of the animal through the water. 



In Platecarpus the cervicals are less slender than -in Clidastes. 

 There are no zygosphenes, or the merest rudiments of them ; 

 the centrum is more transverse. The centra are transversely 

 oval in the dorsal region, more pear-shaped in the pygal, and 

 vertically oval in the caudal. There are six pygals, and their 

 transverse processes are stouter and flatter than in either of the 

 other genera. The vertebrae bearing transverse processes be- 

 hind the pygals are fewer in number than in either of the 

 other genera. The precise number of the vertebrae bearing 

 chevrons cannot be determined, though in all probability the 

 tail does not differ in this respect from that of Tylosaurus. The 

 spines are all regular, there being no dilatation of the tail as in 

 Clidastes, and the obliquity is apparently also not irregular. 

 The chevrons are longer than in Tylosaurus, but not nearly so 

 slender and elongate as in Clidastes. They are all articulated 

 by a rounded head into a small cup-like depression on the under 

 side of the centra back of their middle. 



In Tylosaurus the dorsal vertebrae are yet more transverse in 

 outline and the pygals more pyriform. In fact, in most speci- 

 mens of the pygals the centrum is found almost triangular in 

 shape, the exaggerations due to the crushing, which almost al- 

 ways occurs from above downwards in these specimens, owing 

 to the transverse processes fixing the position of the vertebrae 



