UPPER TRIASSIG SPECIES. 123 



a closely-allied representative species.* In the first place, as was noticed 

 by Mr. Gabb, it is distinctly more compressed laterally than Professor 

 Hauer's figures of A. Ausseeanus, and also differs in having the greatest 

 convexity of the sides of its volutions much nearer the umbilicus. I am 

 aware that these are characters in which the Ammonitoid types vary con- 

 siderably, but the fact that the Nevada specimens are very constant in 

 these characters, while there are also differences in the details of the lobes 

 and sinuses of the septa, as made out by Professor Hauer and Mr. Gabb, 

 leads me to suspect that still other differences would be observed if we 

 had the means of comparing .perfect specimens from the two widely-sepa- 

 rated localities. The differences in the septa alluded to consist in the more 

 conical form of all the lateral lobes and sinuses of the Hallstadt specimens; 

 which also have, according to Professor Hauer's figure, the second lateral 

 lobe distinctly tripartite, instead of bifid at ihe end, as represented by Mr. 

 Gabb. There are likewise other differences in the details of the lobes and 

 saddles, but these might be merely individual peculiarities. I lay no stress 

 on the appearances of very faint traces of longitudinal striae seen on some 

 of our specimens, because others show no indications of them. 



In form, and some of the characters of its septa, our shell is decidedly 

 nearer like Arcestes Barrandei of Laube (Fauna der Schicbt. von St. Cassian, 

 pi. xliii, fig. 2) than it is like the typical A. Ausseeanus, though it differs 

 in having the periphery more broadly rounded, and its sides less convex in 

 the umbilical region, while in the form and proportions of the lobes and 

 sinuses there are differences of perhaps more importance. 



Some time after the foregoing description and remarks were written, 

 Professor Hyatt sent me the following note in regard to this species : 



"It is very distinct in form from A. Ausseeanus, Hauer; witness its 

 greater lateral compression, the scaphitoid or elliptical mode of growth, which 

 is habitual; and the fact that while the adults are. so different, the young 

 are similar in their forms to the adults oi A. Ausseeanus. I think it to be 

 new."— (A. H.) 



Locality and position. — West Humboldt Range, near Cottonwood Canon, 



Nevada (St. Cassian formation). 



* I had at tirsS ouly separated this form here as a variety of A. Atcsseeanus ; but, 

 with the concurrence of Professor Hyatt, I uow view it as a distinct species. 



