Williston.] Mosasaurs. 189 



dorsal vertebrae and fragments of other elements, all belonging 



to one individual." Notwithstanding the apparently functional 



zygosphene, the species evidently belongs in Platecarpus, and 



doubtless would have been located there or in Sironectes had 



the caudal vertebrae been preserved in the type specimen. This 



is apparent from the parietal bone, which, though incomplete, 



was evidently of the peculiar Platecarpus type ; from the shape 



of the frontal, prefrontal, coronoid and vertebral bones. The 



vertebrae differ in no important respect from those of Sironectes 



anguliferus, apparently, and the presence of the zygosphenes 



makes it evident that the species are very closely allied if not 



identical. 



Platecarpus felix. 



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



The larger part of the description of this species is drawn 

 from illusory characters — characters due to the amount of pres- 

 sure and distortion the specimen has received. In no species of 

 the order does the supraoccipital keel, for instance, incline ob- 

 liquely backward, to project beyond the basioccipital condyle. 



"There are eleven teeth on the maxillary, and twelve in the 

 mandible. The great ala of the quadrate is nearly in the same 

 plane as the external margin of the hook, and hence the inner 

 angle is much greater than a right angle. There is a broad, 

 shallow depression on the front face of the ala near the middle, 

 and below this a deep pit on the inner face above the internal 

 angle at the distal end. 



"Width of maxillary at anterior nareal expanse 33mm. 



Width of frontal at posterior margin 115 



Length of quadrate 81 



Length of axis without odontoid process 48 



Length of anterior dorsal vertebra 60 



Platecarpus latifrons. 



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



"The frontals in this species are broad, especially anterior to 

 the orbits, where there is a lateral expansion. Eleven maxillar, 

 twelve mandibular and ten pterygoid teeth. The quadrate has 

 an elongate depression on the back of the ala near the middle, 



