AVillistox.] Mosasaurs. 153 



two facets for articulation with the carpal bones. The outer one 

 is straight and cupped and reaches over nearly the whole extent 

 of the distal surface, that for the pisiform inwardly being merely 

 a notch-like opening between the ulna arid the ulnare carpal. 



Platecarpus coryphaeus. The ulna of Platecarpus is more nearly 

 like that of Clidastes than of Tylosaurus. Both the proximal 

 and distal articular surfaces are placed more obliquely to the 

 axis of the bone and the distal expansion is greater. The ole- 

 cranon expansion is not rounded, but angulated, the roughened 

 cartilaginous surface meeting the free border in a right angle. 

 In Mosasaurus the cartilaginous surface extends out a moderate 

 distance on the projection, but in Clidastes the expansion from 

 the edge of the humeral articulation is rounded and smooth. 

 The distal extremity shows a large, thickened, roughened bor- 

 der on the outer side for the carpal bone ; the outer part thin- 

 ner, less rougheDed, and meeting the free border in a right angle. 

 The boDe, as in Clidastes, is concave above transversely, convex 

 below. 



Tylosaurus prorig&r. The ulna in Tylosaurus is the most slen- 

 der of the arm bones of any genus. Its proximal extremity is 

 a little wider than the distal, and placed obliquely outwards to 

 the axis of the bone. The distal extremity is transverse and 

 but little expanded, its border gently convex. Both the inner 

 and outer free borders are long and gently concave, the great- 

 est narrowing of the bone occurring a little beyond the middle. 

 The olecranon expansion is thin, its border gently convex. 

 The bone is convex from side to side, both above and below. 



Carpus. 



The carpus in Mosasaurus and Clidastes is composed of seven 

 bones, closely articulating, forming a firm and but slightly 

 flexible wrist. In Platecarpus there are four bones, the inner 

 part of the wrist, between the radius and the first and second 

 metacarpals, remaining cartilaginous. In Tylosaurus the carpals 

 exist as a mere rudiment in a single, rounded ossicle, situated 

 at the inner end of the radius and surrounded on all sides by 

 cartilage. 



