166 University of Kansas Geological Survey. 



The distal one, a little smaller, articulates with the two tarsals 

 and the metatarsal. The structure of the hind paddle in Cli- 

 dastes is undoubtedly like that of Mosasaurus. See pi. xxxi, f . 8. 



Platecarpus. The three tarsals preserved in Platecarpus show 

 much resemblance to those of Clidastes. The largest is more 

 nearly circular, with a short, deep, free notch separating the tibial 

 and fibular faces, that of the tibia the larger and thicker. The 

 outer border is thickened and rounded ; the distal and inner side 

 shows two or three facets for the smaller bones. These bones are 

 proportionately smaller than in Clidastes. The proximal one, 

 articulating as in Clidastes, is the smaller, and is broader from 

 side to side than the length ; the other is irregularly five-sided 

 and its diameters nearly equal ; its articulations are as in 

 Clidastes. 



Tijlosaurus proriger. In Tylosaurns there is but a single tarsal 

 bone, small and rounded. It probably represents the largest of 

 the bones of Clidastes and Platecarpus and is set in cartilage op- 

 posite and between the ends of the two leg bones. 



Metatarsals. 



No part of the Mosasaur skeleton is known so imperfectly as 

 the digits of the hind extremity. Of Clidastes, all that is known 

 with certainty are the first and fifth metatarsals and isolated 

 phalanges. Platecarpus and Tylosaurus are better known, still 

 not completely. • 



Clidastes. The first metatarsal in C. velox is broadly and ob- 

 liquely expanded proximally, the angles rounded. The distal 

 extremity is much narrower and the border rectangular, with the 

 outer angle rounded. The inner border is thicker than the outer 

 and more deeply concave, ending more or less acutely. The fifth 

 metatarsal in C. westii is a somewhat disk-shaped bone, with 

 one side concave. The proximal and distal ends are thickened 

 for articulation or cartilage. The proximal border is thin and 

 semicircular. The distal border is thickened and rounded and 

 concave. The bone evidently articulates proximally with the 

 two smaller tarsals. Whether there is a phalanx articulating 

 with it distally is not known, but probably not. 



