Williston.] Mosasaurs. 187 



with plane surfaces froin the proximal articular surface and the 

 external obtuse-angled ridge to the rneatal pit ; the latter, there- 

 fore, not sunk in a depression, as in the other species. 



'• The determination of this species rests on a series of speci- 

 mens from the yellow chalk at a point six miles south of Sheri- 

 dan, Kan. They consist of three vertebrge and fragments of 

 atlas, with numerous portions of cranium and proximal extrem- 

 ity of scapula." 



The determination of this species may be doubtful, but, as 

 only one species in the genus from Kansas antedates it, P. icte- 

 riciis, the name, fortunately, must be eventually acknowledged 

 as of a valid species, should it not be identical with H. ictericus. 

 I suspect that the species is identical with P. coryphseus. 



Platecarpus gracilis. 



Lestosaurus gracilis Marsh, Amer. Journ. Sci., June, 1872. 



"A marked feature in this skull of this species is the superior 

 surface of the parietals, which is small and subtriangular in 

 outline, with the sides incurved. The internal angle of the 

 proximal end of the cpuadrate is much less than a right angle, 

 although the great ala is nearly in the same plane as the outer 

 margin of the hook. This leaves a deep, broad notch between 

 the alar process and the internal angle. There is a deep groove 

 below the meatal pit. The articular ends of the cervical and 

 dorsal vertebrae are transversal oval, with a distinct excavation 

 on the superior margin. Rudimentary zygosphene present. 

 Smoky Hill river. 



" Length of parietal on median line 46 mm. 



Width in front 80 



Length of quadrate 76 



Length of centrum of axis 53 



Xot a single character is given in the above description to 

 distinguish the species, except possibly the smaller size. It 

 may be referred to P. coryphseus with safety. The position of 

 the trochanter of the femur was doubtless due to post-mortem 

 distortion. 



