"Williston.] Mosasaurs. 161 



Tt/Iosaurus proriger. In this species the shaft of the ilium is 

 nearly straight throughout, there being a downward convexity 

 toward the acetabular end. The acetabular end is relatively 

 stout and broad, and the bone more flattened. 



Pubis. 



Clidastes velox. In Clidastes the pubis is the broadest of the 

 pelvic bones. The acetabular end is broadly expanded, with 

 three facets, one on either end of the border, uniting in a slight 

 angle, for ilium and ischium, the third for the acetabulum near 

 the middle and directed outwardly. Below the thickened end 

 there is a moderately constricted, flattened neck, in front of the 

 middle of which is the small pubic foramen. Below this, and 

 directed forward, there is an obtuse, flattened process. Below 

 this process the narrow, flattened shaft of the bone is directed 

 downward into a flattened, somewhat spatulate blade, moder- 

 ately expanded and slightly thickened at the extremity. The 

 bone is shorter than the ilium and of about the same length as 

 the ischium. 



Platecarpus. The pubis in Platecarpus is intermediate in shape 

 between that of Clidastes and that of Tylosaurus. The anterior 

 process exists as a thicker and slightly prominent projection 

 close to the head of the bone. The shaft is placed less obliquely 

 to the long axis of the bone, is thicker and longer, and is con- 

 siderably expanded distally. The anterior border is thinner 

 than the posterior, nearly straight below the process. The 

 posterior border is thick and concave, the thickened portion of 

 the shaft lying on this side. The distal extremity is thinned on 

 the front part, considerably thickened for the ventral symphysis 

 on the posterior. Internally the bone is flattened longi- 

 tudinally. Externally it is concave on the upper part trans- 

 versely. The thickened head for articulation with the ischium 

 is directed upward and backward, the acetabular surface back 

 of the middle is turned outward. The pubic process anteriorly 

 is thickened at its extremity, its oval elongate face for cartilage 

 turned somewhat outward and connected with the cartilagi- 

 nous surface of the upper part by a narrow neck. 



