FOSSILS OF THE JUEASSIC PERIOD. 299 



sliell is most probably not a true JSfatica, and may possibly not belong to 

 the same family; but as the real features of this portion cannot be satis- 

 factorily determined, it remains a matter of doubt. It closely resembles, in 

 general form, shells of the genus Neritopsis except for the smooth surface. 

 It differs also from Neritoma Morris in wanting the peculiar callus of that 

 genus and from Narica Reel, in the smooth surface and want of umbilicus. 

 Formation and localitij. — In a greenish white limestone supposed to be of 

 Triassic age,* northwest of Rawlings Station, Wyoming, and on the East 

 Fork of the Duchesne River, Uinta Range, Utah. Collected by S. F. 

 Emmons, esq. 



*I do not myself believe this to be Triassic, but it has been so referred by 

 others.— E. P. W. 



