134 University of Kansas Geological Survey. 



the presplenial is not a complete one, though, as already stated, 

 undoubtedly admitting of a little lateral motion. Above the 

 shallow ball-and-socket joint the border of the dentary slopes 

 obliquely forward, and is denticulated or fimbriated with outer 

 and inner laminae interdigitating with similar processes of the 

 coronoid and surangular. 



In Clidastes tortor (pi. xxin) the arr augment of the bones 

 on the inner side of the posterior part of the mandible is very 

 different from what it is in Platecarpus. The coronoid is much 

 longer and more prominent, and sends down a broad internal 

 plate to overlap the upper part of the splenial, completely in- 

 closing, in this region, the anterior part of the articular. The 

 surangular is narrower, the anterior end more pointed. The 

 dentary is more slender and there are eighteen teeth. 



In Tylosaurus the structure is much more like that of Plate- 

 carpus than that of Clidastes. The coronoid and splenial are 

 broadly separated, the angular exposed. The coronoid is 

 longer and more prominent relatively than in Platecarpus. The 

 dentary projects more in front of the first tooth. There are 

 thirteen teeth. 



In Brachysaurus (pi. xxn) the mandible is stouter and broader. 

 The coronoid is more prominent than in Platecarpus and Tylosau- 

 rus. The dentary is extraordinarily concave on its upper border. 

 It is remarkably short and stout and has twelve or thirteen 



teeth. 



Vertebrae. 



The vertebrae in the known forms of the Mosasaurs vary in 

 number from one hundred and seventeen to possibly one hun- 

 dred and fifty, though none are known from the Kansas Cre- 

 taceous with more than one hundred and twenty. It is not at 

 all improbable that the number of the caudals, or even of the 

 precaudals, may vary to a slight extent in different individuals 

 of the same species. Much reliance has been placed upon the 

 form of these bones in specific classification, but I doubt very 

 much whether such characters are generally reliable, largely for 

 the reason that most of the Kansas specimens from the Nio- 

 brara Cretaceous show more or less distortion from pressure 



