142 



animal more nearly allied tothc true Mosasauriis. The place of P/aiecarpus 

 is evidently between Clidastes and Mosasaurus; the palatine bones being those 

 of the former, and the vertebral articulations being identical with that charac- 

 teristic of Mosasaurus. In all of the sjiecies, traces of the zygosphen appear; 

 but in the H. coryphcRus, Cope, the rudiment amounts to a short process 

 directed forward at the base of each anterior zygapophysis. 



The species known, as yet, are of medium size in the order. 



The species P. mudgei and P. tectulus resemble each other in the form 

 of their quadrate bone, and are referred to this genus provisionally only. 

 The P. simus, Marsh, resembles the P. crassartus, Cope, but differs in the 

 depressed, instead of circular, articular faces of the dorsal vertebrae. 



It is probable that this genus had a considerable geographical distribu- 

 tion. P. tympaniticiis has been found in A.labaraa ; and I suspect that other 

 species from other localities belong to it. 



Platecarpus coryph/Eus, Cope. 



Characters. — Cervical and dorsal vertebrae with the articular surfaces 

 depressed transverse, slightly excavated above for the neural canal. The dia- 

 pophyses not continued inferiorly to the rim of the cup on the cervical ver- 

 tebrse, and not receiving from it a cap of articular cartilage. Occipital crest 

 much elevated. Quadrate bone small, the meatal pit depressed between 

 bounding ridges above and below. Rudimental zygosphen not uniting into a 

 keel above. Teeth slender, less curved than in P. ictericus. 



Description. — This species is chiefly based on one specimen, which 

 includes the greater part of the cranium, and seventeen vertebrae, with ribs. 

 Isolated portions of other individuals were also found in the same region of 

 country. 



The disproportion between the diameters of the cervical and dorsal ver- 

 tebrae is more marked here than in the species of Clidastes. The centra are 

 also less elongate, though with larger diameter. The cranium is relatively 

 much smaller ; the teeth absolutely smaller, though the quadrate bones are 

 of equal size. The genei'al character of the species is stouter, but less 

 strongly armed, and less elegantly built. 



The hypapophysis of the atlas has a short small keel below. The neu- 

 ral spine of the axis is elongate, but less so than in the two Clidastes, trun- 

 cate behind, with a median groove, into which the anterior kocl of the neurnl 



