160 University of Kansas Geological Survey. 



free, flattened extremity, directed obliquely upwards and for- 

 wards in life. Its union with the vertebral column is very weak, 

 if there is any union at all. The end probably lies in relation 

 with the first non-costiferous transverse process, though prob- 

 ably without any connection, inasmuch as none of these proc- 

 esses shows any special indication of ligamentous attachments. 

 The pubis is flattened spatulate at the distal extremity, and, if 

 it meets the fellow of the opposite side, the union must be 

 slight. The ischium, on the other hand, shows a broad and some- 

 what flattened distal extremity of considerable thickness, espe- 

 cially in Platecarpus. Doubtless there is a true symphysis here, 

 forming a complete pelvic girdle. The pelvis is, relatively to 

 the pectoral girdle, the weakest in Clidastes and strongest in 

 Tylosaurus. It is, however, absolutely the strongest and best 

 developed in Platecarpus of any of the three genera. 



Ilium. 



Clidastes velox. The ilium in this species is a slender, some- 

 what flattened, rod-like bone, gently concave along the upper 

 border, for most of its length, and correspondingly convex be- 

 low. The extremity is flattened, thin, and somewhat roughened. 

 At the acetabular end the bone is expanded into a thickened 

 head, the anterior border curving suddenly downward, while 

 the posterior or upper is gently convex. It has two facets 

 of nearly equal length, the posterior one for union with the 

 ischium, the anterior for the pubis ; a third cupped surface 

 forms part of the acetabulum. The side to which the bone be- 

 longs may be determined by the outward direction of this face. 

 The articulations are more roughened, as they are in all the 

 bones of the girdles and extremities, indicating a thicker cover- 

 ing of cartilage. 



Platecarpus coryphaeus. The ilium in this species is more 

 thickened and stout than in Clidastes, and the facets at the 

 proximal extremity less well marked. The upper or posterior 

 border is gently convex, and not sinuous, the convexity greater 

 near the acetabular end. 



