F. B. Loomis — Wew Mosasaur from the Ft. Pierre. 557 



1 

 Genera Horizon 



Praernaxillo- 

 nasals 



Maxilla 



Quadrate 



Caudal 

 vertebrae 



i 

 Niobrara 

 Ulidastes to 



1 Ft. Pierre 



Rostrum sharp, 

 of mod. length : 

 median ridge 

 separate teeth 



15-16 teeth 



small : suprastape- 

 dial process long 

 but not fused. 



7 pygals : chev- 

 rons fused: tail 

 broadened. 



I 

 Tylosaurus Niobrara 



Rostrum very- 

 large and long: 

 smooth below 



13 teeth 



medium : suprasta- 

 pedial process 

 small, not fused. 



6 pygals : chev- 

 rons artic- 

 ulated. 



tfosasaurus Niobrara 



Rostrum me- 

 dium size and 

 short : median 

 groove. 



14 teeth 



medium : suprasta- 

 pedial process 

 short, not fused. 





3rachysaurus < Ft. Pierre 





11 teeth 



moderately large : 

 suprastapedial pro- 

 cess long, and fused. 



chevrons fused : 



-'latecarpus 



Niobrara 



to 

 Ft. Pierre 



Rostrum small, 

 obtuse, and 

 short : smooth 

 below. 



12 teeth 



large : suprastape- 

 dial process large, 

 not fused. 



5 pygals : chev- 

 rons artic- 

 ulated. 



Amherst 

 specimen 



Ft. Pierre 



Rostrum small, 

 obtuse, and 

 short : median 

 ridge behind 

 teeth. 



11 teeth 



large : suprastape- 

 dial process very 

 large, not fused. 



5 pgyals : chev- 

 rons artic- 

 ulated. 



gence. I have therefore placed this in the genus Platecarpus* 

 though I realize that it represents an intermediate relationship. 



Platecarpus has a couple of rather doubtful Ft. Pierre 

 species, ? P. latispinis Cope, based on one cervical and five 

 dorsals, peculiar in the considerable length of the diapophyses, 

 and of a size about 25 per cent larger than P. bachycephalus : 

 and P. tectulus Cope, based on a number of cervical and dor- 

 sal vertebrge of unusually small size. The Amherst material 

 can not be associated with either of these. 



The skull as a whole is remarkable for its extreme brevity 

 and width. As found it is crushed from above downward, to 

 which is due the abnormal position of the jugal bones (see 

 fig. 1). The nares are well forward, short and relatively wide. 

 The orbits are short and wide. The exoccipital region is pro- 

 longed considerable back of the condyle, much further than 



*Iuse the term Platecarpus for this genus as the one in common use. 

 Hay, in his Bibliography of Fossil Vertebrates of North America 1902, uses 

 Marsh's name, Lestosaurus. The priority is dependent on the final deter- 

 mination of dubious types, which has not yet been done. See Williston, 

 Univ. Geological Survey of Kansas, vol. iv, p. 178. 



