Williston.] Mosasaurs. 113 



border, as is found in the following forms. There is usually a 

 small articular surface situated far back on the inner side, at 

 the margin, of the greater concavity, for union with the ecto- 

 pterygoid. 



Platecarpus. PI. xxiv, f. 5 ; pi. lxiii, f . 3. In this species the 

 jugal is a stouter bone than in Clidastes, but not as stout as in 

 Tylosaurus. It is somewhat L-shaped in form, with the hori- 

 zontal arm slender and curved downward. The upright arm is 

 flattened obliquely, and deeply excavated above on the outside 

 for the jugal process of the postfrontal, the excavation reaching 

 more than half the distance to the angle of the bone. Its pos- 

 terior superior angle is thickened, and unites with the prosqua- 

 mosal. Where the bone begins to curve forward there is a 

 prominent tubercle on the outer hind margin, as if for liga- 

 mentous attachment, and the bone is channeled obliquely in- 

 ward in front of it. The anterior branch is dilated and flattened 

 at the extremity, and excavated into a groove on the under side 

 for articulation with the posterior end of the maxilla. At the 

 posterior end of the horizontal arm in front of the angle, on the 

 inner side, there is a depressed sutural surface, of variable 

 length, for union with the ectopterygoid. 



Tylosaurus. In Tylosaurus the jugal is a stouter bone than 

 in either of the other genera. It is bent more nearly in a right 

 angle than in Platecarpus, and the anterior prolongation is less 

 slender, the end less dilated. The upper ramus is broader and 

 flatter, and less deeply excavated for the jugal process of the 

 postfrontal. The angle below is broadly rounded, the margin 

 is thinner, and not produced into a tubercular process ; instead 

 of which, there is, on the outer side, a roughened depression of 

 considerable size for the insertion of a strong ligament or tendon. 

 The vertical arm is set more obliquely and the surface is con- 

 cave transversely. On the inner border there is a roughened, 

 elongate spot for the transverse bone. The horizontal arm is 

 flattened and much stouter than in Platecarpus. The beveled 

 and grooved articular surface for the maxilla is on the inferior 

 inner side, and extends far back. The bone runs to a narrow 



