Williston.] Mosasaurs. 163 



this the surface is concave above, more strongly so below. The 

 blade of the bone below on the outer side is correspondingly 

 convex. 



Femur. 



Clidastes. The femur of Clidastes is a relatively small, rather 

 slender bone. The head is moderately expanded, transverse, 

 with an oval, convex, articular surface. The trochanter, situ- 

 ated on the tibial side, is stout, prominent, directed nearly up- 

 ward, with the large oval extremity tipped with cartilage, and 

 narrowly separated from the articular surface of the head by a 

 slender neck. The stout, rounded ridge of the trochanter ex- 

 tends below the middle of the bone. The distal extremity is 

 more expanded than the proximal. It has a broad, thickened 

 articular surface for the tibia, and a smaller, thinner and ob- 

 lique one for the fibula. The tibial border is thickly rounded 

 and gently concave throughout. The shorter and thinner fibu- 

 lar border ends in the thinner rounded lateral expansions above 

 and below. 



Platecarpus. The femur in Platecarpus is much like that of 

 Clidastes. The trochanter is hardly as large, the extremities a 

 little more dilated, the fibular angles a little less rounded. The 

 distal extremity is more expanded. 



Tylosaurus proriger. In Tylosaurus the femur is more slender 

 than in either of the other genera, but the differences from 

 Platecarpus are not great. The lower extremity is more ex- 

 panded and convex, the sides more deeply concave, of nearly 

 equal depth and length. The sides meet the lower border in 

 an acute angle, lacking the slight convexity of Platecarpus and 

 the marked convexity of Clidastes. The head of the trochanter 

 stands a little lower down on the bone, and its coarse pitting 

 for cartilage is less distinctly separated from the articular sur- 

 face of the head by the smooth neck. The bone is nearly bilater- 

 ally symmetrical, except the lateral position of the trochanter. 

 In both Clidastes and Platecarpus the bone is markedly unsym- 

 metrical, the greater flattening and deeper concavity being on 

 the fibular side. 



13— iv 



