Willi ston.] Mosasaurs. 103 



four teeth, the anterior pair smaller than the maxillary teeth 

 and approximated. A median ridge separates the teeth, and is 

 continued into a stronger one back of them. Between the two 

 narial openings the inferior border is thin, and is wholly or 

 largely hidden by the approximated vomers. The bone articu- 

 lates with the maxilla?, vomers, and frontal. 



Jlosasaurus horridus. The tip of the rostrum is rather more 

 obtuse than in the previous species, but not as much so as in 

 the following, projecting a short distance beyond the teeth. 

 The portion in front of the maxillae is about as long as wide, 

 nearly semicircular in cross-section, with a shallow longitu- 

 dinal groove in the middle above, in place of the obtuse carina. 

 This groove reaches to about the beginning of the nares. The 

 lateral margins above are very long and oblique, resembling in 

 this respect Tylosaurus more than Clidastes. The opening of 

 the nares is opposite the fifth maxillary tooth. 



Platecarpus coryphseus. PI. xxvi, ff. 2, 3. The premaxillary 

 is short and obtuse, differing markedly from the other genera 

 in not projecting at all beyond the teeth, the tip often with a 

 distinct depression, instead of a convexity or cone. It is 

 smoothly convex above, without median ridge or convexity. 

 The sutural union for the maxilla? runs nearly obliquely from 

 the dental border back to the anterior end of the nares, very 

 unlike what it is in Clidastes. The internarial process is nar- 

 row and oval in cross-section for a short distance before the 

 middle of the nares. Posteriorly it widens uniformly into a 

 thin, flattened plate, the conjoined nasals, which overlie the 

 anterior prolongation of the frontal, the suture nearly opposite 

 the posterior end of the nares. The nares are much shorter 

 relatively than in Clidastes. 



Tylosaurus. The premaxillo-nasal in Tylosaurus is one of the 

 largest elements of the skull, and is very characteristic of the 

 genus. It forms a long, obtuse projection in front of the teeth, 

 the edentulous portion being considerably longer than the den- 

 tulous. It is smooth and rounded, nearly circular in cross- 



