162 University of Kansas Geological Survey 



Tylosaurus proriger. The pubis of Tylosaurus may be at once 

 distinguished from the same bone in the other genera by the 

 entire absence of the pubic process. The bone also is relatively 

 shorter and less expanded distally. The front border has a 

 gentle convexity from the iliac angle to beyond the middle, 

 whence the margin is more nearly straight to the extremity. 

 The iliac and ischiac borders occupy each about half of the upper 

 end and meet in a slight angle. The posterior border is gently 

 concave or nearly straight throughout nearly the whole extent. 



Ischium. 



Clidastes velox. The ischium of Clidastes has a moderately 

 constricted neck, placed at a strong angle with the long axis of 

 the bone ; a prominent, thinned tuberosity directed directly 

 backward, near the upper end of the bone, and separated by a 

 shallow concavity from the acetabular surface ; and a stout, 

 thickened and expanded symphysis below. The articular sur- 

 face for the pubis is nearly parallel with the long axis, and 

 nearly at right angles with that for the ilium. 



The Platecarpus ischium differs markedly from that of either of 

 the other genera. The ischial tuberosity is small, the pubic face 

 is more oblique to the long axis, and the distal extremity is greatly 

 dilated into a long,. angular projection anteriorly. The distal 

 extremity when seen from the end presents a shallow, V-shaped 

 figure, with the pubic border much longer and thinner, the pos- 

 terior border very stout and thick. The bones evidently meet 

 in a very firm and broad symphysis. From without the bone 

 presents a long, obtuse ridge with the sides shallowly concave. 

 On the inner side there is a corresponding depression. 



Tylosaurus proriger. In Tylosaurus the bone is less stout than 

 in Platecarpus, and resembles that of Clidastes more nearly, dif- 

 fering chiefly in the broader blade below, with a broader ex- 

 tremity and less constricted shaft above. The spinous process 

 is more pronounced and is tipped with cartilage. A rather 

 pronounced ridge runs from the uppermost, posterior angle 

 downwards on the inner side, to become continuous with the in- 

 flected anterior border of the bone on the lower half. Back of 



