UPPER TRIASSIO SPECIES. 105 



section, and the other seems to have been almost exactly cylindrical, and 

 is somewhat compressed, though evidently by accidental pressure. It also 

 shows some appearances of a slight constriction at the aperture, and has, in 

 the cast, a raised line along the ventral side, as we often see in palaeozoic 

 species. As there are no appearances of septa, I have sometimes suspected 

 that it may possibly be the cast of a Belemnite ; but it seems not to be. 



Not having had an opportunity to compare good specimens of 0. 

 Blalcl with European species from the same horizon, I have formed no 

 opinion in regard to its relations to the latter, and merely refer the form 

 under consideration doubtfully to Mr. Gabb's species, "iDccause it came from 

 the same region and the same geological position; while the specimens 

 present no characters inconsistent with the conclusion that they belong to 

 the species described by Mr. Gabb. 



Locality and position. — Eidgc above Cottonwood Canon, West Hum- 

 boldt Range; Upper Trias. 



AMMONITOID FORMS OF THE UPPER TRIAS OF NEVADA. 



In examining the shells of the above-mentioned types, in Mr. King's 

 collection from the Upper Trias, or possibly in part from the Lower Lias 

 of Nevada, it soon became evident that none of them would fiill properly 

 into the genus Ammonites as the latest methods of classification Avill require 

 that group to be restricted. It was also equally manifest that the same 

 principles of generic limitation would require the establishment of new 

 genera for the reception of some of the species. Having neither the neces- 

 sary material at hand, nor the time nor inclination, merely for the classifica- 

 tion of a few species, to enter upon the study and revision of the whole 

 group of Triassic and Liassic Ammonitoid types, I proposed, after separat- 

 ing and writing out full descriptions of the species, to send the specimens to 

 Professor Hyatt (who has long made an especial study of these older forms 

 of this great group of extinct Molluscs), with the view of having them com- 

 pared with the splendid series of European forms in the Cambridge Museum 

 of Comparative Zoology ; the understanding being that he should name and 

 describe the new genera, and that I should describe the si)ecies and refer 



