AVilliston.] 3Iosasaurs . 147 



Humerus. 



This bone is characterized by the absence of a distinct neck, 

 by its constrictions and transverse dilatation distally. There 

 appear to be two distinct types, characterized by the tuberosi- 

 ties of the distal end. 



Mosasaurus horridus. The humerus in this species is a mas- 

 sive bone, agreeing best with the humerus of Clidastes. The 

 greater diameter of the proximal articular surface is antero- 

 posterior, instead of transverse. The slightly pitted, trans- 

 versely gently concave surface is crescentric in outline, with 

 the concavity facing the radial border. At the ulnar side proxi- 

 mally, there is a stout, massive tuberosity, directed upward 

 and inward, and rising about an inch above the articular sur- 

 face. It is subovate in cross-section, its end convex, and the 

 surface cartilaginous. The stout pectoral ridge is scarcely dif- 

 ferentiated from the body of the bone, being very broad. It is 

 situated nearly in the middle of the bone, its proximal end 

 forming the inferior end of the large crescent already described. 

 The distal border has the broad, thickened surface for articula- 

 tion with the radius placed obliquely, looking to the radial side 

 and also dorsally. At its outer end there is a stout, styliform, 

 flattened process directed outwards in the horizontal plane of 

 the bone, its oval extremity showing a cartilaginous surface. 

 The ulnar condylar process is stout and projects on the palmar 

 side of the bone. Its distal surface looks inward and some- 

 what downward. 



CHrfastes velox. In this species the proximal end of the bone 

 is transverse, but less regular in outline than in the following 

 genera. The glenoid articular surface is situated near the 

 middle transversely, its surface concave in both directions. 

 The articular or cartilaginous surface is continuous to the in- 

 ner angle of the bone. This portion of the bone, which in Mo- 

 8a8awus is raised prominently into a tuberosity, is only a little 

 prominent in Clidastes, forming the inner two-fifths of the mar- 

 gin of the bone, and ending in a thin border. Possibly in life 

 the resemblance between the two genera in this part may have 



12— iv 



