Williston.] Mosasaurs. 107 



this eniargination the inner upper part of the bone overlies 

 broadly the sides of the prefrontal. Posteriorly this border ends 

 in a small, tongue-like projection on the prefrontal, outside of 

 which the margin is continuous with the pointed posterior ex- 

 tremity of the bone. There are twelve teeth. The articular 

 surface for the vomer is situated further back than in Clidastes, 

 nearly over the third tooth, and the emargination for the pala- 

 tine foramen is much larger, reaching to beyond the fourth 

 tooth. The surface for union with the palatine is shorter. 



Tylosaurus. The maxilla of Tylbsaurus is intermediate in 

 length and breadth between those of Clidastes and Platecarpus. 

 The maxillary suture extends as far back as the sixth tooth, 

 and is much longer than in either of the other genera. Back 

 of this the emargination for the anterior part of the nares is 

 less deep than in Platecarpus. The prefrontal process is rather 

 larger than in Platecarpus, the posterior extremity more slender. 

 The sutural surface for the vomer is situated further back than 

 in Platecarpus; that for the palatine is nearly the same. There 

 are thirteen teeth. 



Brachysaurus overtoni. PI. xxn, f. 1. The maxilla in Brachy- 

 saurus is very massive and stout. The border for the pre- 

 maxilla is very short, extending back as far as the third tooth 

 only, the vertical portion only a little shorter than the oblique 

 portion. There are ten, probably eleven, teeth, implanted on 

 prominent, broad bases. 



Prefrontals. 



Clidastes. The prefrontal is the most characteristic bone of 

 the Clidastes skull, varying somewhat in the different species. 

 It is elongate and flattened on the upper surface, with stria- 

 tums and markings like those of the frontal on whose plane 

 the horizontal portion is. It projects strongly over the orbits 

 in front, serving functionally in place of the supraciliare of the 

 Varanus skull. *The inner border is concave to correspond with 

 the border of the frontal. The outer border is free, nearly 

 straight, and lightly rugose for nearly half its length, protruding 

 horizontally. On the inner side anteriorly the bone forms a 



