AVilliston.] Mosasaurs. 115 



a more or less sharp ridge, but they are never pleurodont or pro- 

 tected by a parapet, a character that has been given for distin- 

 guishing the genus Edestosaurus from Clidastes, but which is of 

 no value whatever. 



Mosasaurus horrid us. The pterygoids in the type specimen of 

 this species are in position and undistorted, while those of Cli- 

 dastes are invariably more or less distorted. They are evidently 

 very closely alike, and present characters that readily distin- 

 guish the bone from the same in the other subfamilies. The 

 anterior end terminates in a broad plate on the inner side, which 

 slopes markedly toward the middle. There are eight teeth in a 

 single curve, whose concave side is internal and reaching from 

 before the posterior end of the palatine, and opposite the last 

 maxillary tooth, to the base of the basisphenoid process. The 

 teeth back of the palatine stand very nearly on the outer edge 

 of the bone on a convex surface, and are not all pleurodont. 

 The basisphenoid process is longer than in Clidastes. The ecto- 

 pterygoid process is placed like that of Clidastes ; that is, 

 obliquely. Its posterior border is continuous with the inferior 

 border of the quadrate process, while the anterior is continuous 

 with the superior interior border of the body of the bone, inclos- 

 ing a long, shallow groove between its base and the part on 

 which the teeth are inserted. In Platecarpus and Tylosaurus the 

 process is nearly horizontal, and its anterior border is continu- 

 ous with the outer border of the bone, or nearly so. The bone 

 differs from that of Clidastes, in the lesser number of teeth (eight) 

 and in the less expanded inner side anteriorly. 



Platecarpus . Pl.xxiv, f. 1. The pterygoid of Platecarjms has 

 ten teeth arranged in the form of a reverse curve. The teeth 

 extend into the base of the basisphenoid process, which lies 

 closely in the depression on each side of the lower surface of 

 the basisphenoid. The quadrate process is nearly vertically 

 flattened or gently concave on the inner side, narrower and 

 stouter at the base, obliquely truncate and roughened at the 

 tip for union with the quadrate. The under border of the 

 dilated portion is strongly convex. Nearly opposite the base 



10— iv 



