130 University of Kansas Geological Survey. 



above the upper end of the meatal notch. The suprastapedial 

 process is very short and broad, and its posterior face only a 

 little constricted above. It extends only about a third of the 

 length of the bone. On the posterior surface there is a strong 

 rugosity a little below the middle of the bone, continued into 

 an oblique low ridge that runs to the outer inferior angle. The 

 ear cavity is more shallow and much longer from above down- 

 ward than from side to side ; the outer border but little convex. 

 Its wall is thicker than in Platecarpus, where the bottom of the 

 concavity is very thin. The inner angle above is stout and 

 much produced, but the concavity between it and the alar 

 process is long and not very deep. The inner border of the 

 articular surface is rather deeply concave. 



Mandible. (Pis. xxu, xxn.) 



The mandible is one of the most peculiar parts of the Mosa- 

 saurian skull. It is proportionally very large and stout, and 

 exceeds the skull proper in length. Back of the middle there is 

 an imperfect joint between the splenial and presplenial bones. 

 This joint admitted motion in both lateral and vertical direc- 

 tions, though chiefly in the former. The motion, even laterally, 

 could not have been great, since a thin plate of the angular 

 extends forward within the cavity of the presplenial. Doubtless 

 this thin extension was elastic, permitting some inward flexion 

 of the dentary, and helping to restore extension. The extension 

 of the articular back of the cotylar cavity was never great, 

 but the broad, stout posterior part of the mandible evidently 

 indicated powerful muscles for the seizure and holding of the 

 prey. The following descriptions of the different elements is 

 from a jaw of Platecarpus coryphseus, a figure of which from the 

 inner side is given in plate xxu. 



The articular forms part of the cotylar cavity — a little less 

 than half — the sutural line running obliquely from before back- 

 wards and outwards. The surface in this part is concave from 

 side to side. The portion back of the cavity, separated by an 

 obtuse ridge, is placed obliquely, though in most specimens it 

 is crushed flat so as to lie vertically, while in reality it is more 



