AVillistox.] Mosasaurs. 151 



Clidastes velo.v. In Clidastes velox the radius is almost fan- 

 shaped. The proximal end is thickened, ovate in shape, the 

 greater thickening on the outer side. The articular surface is 

 smooth and gently concave for close union with the humerus. 

 Beyond the head, at about the proximal third, the bone is much 

 contracted, the width here being about one-third of that in its 

 widest place. On the distal end the bone is much expanded, 

 forming more than half of a circle, with the carpal articular 

 border a chord. The outer margin is short and deep, the border 

 thinned. The inner border is thicker and about a half longer 

 than the outer, its distal end forming an acute angle with the 

 inner end of the carpal surface. The carpal border forms more 

 than a third of the distal periphery. It is thickened, concave 

 and smooth on the surface, and placed very obliquely to the axis 

 of the bone — about forty-five degrees. The remainder of the 

 distal border is thin, and gives attachment to cartilage. I can 

 give no rule by which the bones of the two sides may be dis- 

 tinguished, save perhaps that the thickening of the proximal 

 end seems to be less uniform on the dorsal side, its greatest 

 convexity in cross-section being on the outer side. 



Platecarpus. In Platecarpus the shape of the radius is much 

 more like that of Clidastes than of Tylosaurus. The bone is 

 broader and natter, the proximal portion less constricted, the 

 distal border more uniformly a half circle — that is, the carpal 

 border is not so distinctly a chord of a circle — and the curva- 

 ture transversely is less strong. The articular part of the distal 

 extremity is only a little thicker than the free cartilaginous 

 border. The whole surface of the border is, moreover, coarsely 

 roughened, showing an intervening attachment of fibro-cartilage. 



Tylosaurus. The radius of Tylosaurus, like the other paddle 

 bones of this genus, is elongate and narrow. The proximal end 

 has an oval articular face at right angles to the long axis of the 

 bone, for articulation with the humerus. The shaft is only mod- 

 erately narrowed to near the middle of the bone. The distal 

 extremity is about a half wider than the proximal. On its 

 inner side it is thickened for more than half its width ; the 



