154 University of Kansas Geological Survey. 



Mosasaurus horridus. There are four carpal bones in the prox- 

 imal row, which may be designated the radiale, mediale, ulnare, 

 and pisiform. The radiale is trapezoidal in shape, the inner 

 end narrower than the outer. Its proximal articulation is the 

 longest and thickest ; it articulates closely with the radius, but 

 does not extend quite to the inner end of the distal border of 

 that bone. The distal border, nearly as long and thick, is 

 straight and turned somewhat more outwards ; it articulates 

 with the broad first metacarpal. The outer, free border is 

 rounded and thinner than any of the others. The inner border, 

 less thick than the long borders, meets the distal border in a 

 rounded rectangle, and articulates with the rounded bone of the 

 second row. Between this and the upper border the margin is 

 rounded, where it meets the mediale for a short distance. The 

 mediale is the next in size of the carpal bones, but is thinner 

 than either the radiale or ulnare. It has a free, thin, concave, 

 proximal border, extending from the distal, opposing angles of 

 the forearm bones. The distal border, nearly parallel with the 

 chord of the free border, is for articulation with the middle bone 

 of the second row. Its outer border has three articular surfaces 

 in this species ; the two proximal ones rounded and indistinctly 

 separated, for articulation with radiale and radius ; the distal 

 one, as long as the other two together, is for articulation with 

 the third of the distal row. On the ulnar side there are two 

 articulations, the proximal one the thickest of all, but short, for 

 union with the ulnar ; the other, long one, for union with the 

 ulnare. The ulnare is a stout, nearly square bone, nearly or 

 quite as large as the mediale. Its outer border, for union with 

 the thinned mediale, is abruptly thinned. Its proximal and 

 distal borders are very thick ; the former slightly convex from 

 above downwards, the latter gently concave. The distal one is 

 a little the longer, and articulates with the third distal carpal ; 

 the proximal with the ulna. The outer side is rounded and has 

 two small articular facets, the proximal one, the larger, form- 

 ing an entering angle with the ulna, for the pisiform ; the distal, 

 smaller one, for articulation with the fifth metacarpal. In the 

 distal row there must have been three carpals, only one of which 



