"Williston.] Mosasaurs. 149 



ued as a thin margin into the stout ulnar condyle, whose sur- 

 face looks downwards and distad. The pectoral process is very- 

 small, is oblique in position, as in Platecarpus, and is situated 

 close to the radial side of the bone ; its ridge is thin . The dor- 

 sal surface is markedly concave from side to side ; the palmar 

 surface, flattened and concave. The specimen described, like 

 that of Mosasaurus has not been subject to the compression so 

 common among the Kansas specimens. Its resemblances are 

 apparently greater to Platecarpus , though very different in the 

 convexity and angularity of the proximal end. See plate lxii. 

 Platecarpus coryphseus. The humerus in this genus is the most 

 expanded of any of the forms known to me, its width distally 

 being nearly as great as the length. The proximal border is 

 nearly straight, rransversly or gently concave along the middle, 

 the margins thinner and rounded. The distal border is much 

 expanded and broadly convex, forming nearly the half of a 

 circle. The thickened articular surface for the radius looks a 

 little obliquely outward ; proximad to the thickened portion the 

 border forms a gently convex outline, its chord nearly parallel 

 to the longitudinal axis of the bone ; this portion is thinned 

 and the surface covered with cartilage. In no other form, save 

 Brachysaurus, is the structure similar here. The ulnar side is 

 more expanded than the radial, its convex border nearly longi- 

 tudinal. The surface here, however, is thickened to form the 

 tuberosity, which looks downward. The tuberosity is not very 

 stout, and is connected with the ulnar articular border by a 

 thinner margin, which is not emarginated. Both the radial and 

 ulnar borders of the bone are deeply emarginated, the bone be- 

 ing much constricted above the middle. The ulnar border is a 

 little, though not much, longer than the radial. The pectoral 

 process is situated more nearly in the middle of the bone than 

 in any of the other forms. It is obliquely inclined toward the 

 radial side, and compressed from side to side, the ridge blend- 

 ing with the surface a little below the middle of the bone. Its 

 cartilaginous surfaces reaches through more than half of its 

 extent, and is either entirely separated from the cartilaginous 

 surface of the head or is only narrowly connected. The proc- 



