157 



Measurements. 



M. 



Length of the centrum of the anterior vertebra 0. 064 



Diameter of the ball, vertical 0.030 



Diameter of the ball, transverse 0.039 



Length of the posterior 0. 064 



Diameter of the ball, vertical 0. 030 



Diameter of the ball, horizontal 0. 043 



Expanse of the anterior zygapophyses 0. 055 



The first vertebra was found by the writer at the foot of a bluff on the 

 lower part of Butte Creek; the second was procured by Prof. B. F. Mudge, 

 from a point one mile southeast of Sheridan, near the North Fork of the 

 Smoky River. 



It is this species that I compared with the Mosasaurus dep>ressus, Cope, 

 in a report on the collection made by Professor Mudge (Proceedings of the 

 American Philosophical Society, 1871. p. 168). The size is similar, but the 

 form of the articular surfaces of the vertebrae is very different. 



Platecarpus mudgei, Cope. 



The characters distinguishing this saurian are the following : Vertebrae 

 without rudimental zygosphen; quadrate bones with plane surfaces from the 

 proximal articular surface and the external obtuse-angled ridge to the meatal 

 pit; the latter, therefore, not sunk in a depression as in the other species. 



The determination of this species rests on a series of specimens from the 

 yellow chalk at a point six miles south of Sheridan, Kans. They consist 

 of three vertebrae and fragments of atlas, with numerous portions of cranium 

 and proximal extremity of scapula. 



The parts of cranium preserved are the frontal bone without the anterior 

 extremity, and with the adjacent parietal almost complete; parts of the basi- 

 sphenoid ; the suspensorium ; the ossa quadrata; and the greater part of the 

 surangular. The frontal is flat, with thin edge, longitudinally hollowed on each 

 side of the median line, which is marked by a low but acute keel. There is 

 an abundance of foramina and delicate grooves on the surface, and posteriorly 

 elevated striae, which converge to the median keel. The median square ex- 

 tension of the border of the parietal is in advance of the lateral portion of the 

 same, and not behind it as in Clldastes propython. The fontanelle is large. 

 A marked feature is that the parietal crests unite into a low median ridge a 

 short distance behind the fontanelle, and are not, as in C. propython, separated 

 by a horizontal plane. The sutures of the bones forming the side of the brain- 



