Williston.] Mosasavrs. 145 



margin on the outer side is produced into a sharp, prominent 

 lip. 



Mosasaurus horridus. An imperfect scapula of this species 

 apparently closely resembles the same bone in Clidastes. It is 

 less distorted and crushed, so that the neck is less flattened, 

 and the lip of the glenoid articulation is produced into a sharp, 

 prominent ridge. 



Platecarpus coryphseus. The scapula in Platecarpus has greater 

 vertical height than in Clidastes and less antero-posterior elon- 

 gation. The superior border is much more strongly convex, 

 the greatest convexity being at the uppermost part. The pos- 

 terior part is not as strongly produced, the angle not as 

 acute. The free, thin upper border is much shorter or reduced 

 to a short convexity, the concave border below it much longer 

 and thicker. The anterior inferior border is concave as in 

 Clidastes, ending in a sharp margin somewhat deflected on the 

 inner side, the thinning due to a broad groove on the outer 

 surface. In some specimens the border is concave, in others 

 straight from the anterior angle to the smaller roughening 

 in front of the coracoid articulation. Around the lip of the 

 coracoid articulation there is a considerable ligamentous rough- 

 ening, as in Clidastes. 



Tylosaurus dyspelor. The scapula of Tylosaurus closely re- 

 sembles that of Platecarpus, but is, relatively, both to the size 

 of the coracoid and to the skeleton, a much smaller bone, its size 

 in T. proriger, of twenty-four feet in length, being absolutely 

 smaller than in P. coryphaeus, of thirteen feet in length. The 

 bone in Tylosaurus is less expanded vertically and antero-pos- 

 teriorly than in Platecarpus. The posterior emargination is 

 shorter, the superior border thicker. 



Coracoid. 



Clidastes velox. The coracoid is a broad, thin, fan-shaped 

 bone, flattened on the exterior surface, concave on the inner. 

 The head, like that of the scapula, is divided by an oblique 

 ridge into unequal facets, meeting in an obtuse angle. The an- 

 terior and large one for the scapula is concave, and directed ob- 



