6 AMERICAN JOURNAL 



the former, Pecten acutiplicatus, Meek, is by no means rare. 

 In most cases, by carefully studying the lithological characters 

 of recognized forms of the two formations, the new species can 

 be assigned without difficulty to their proper group, though in 

 some instances I am still in doubt. 



The present paper is intended to be merely preliminary, it 

 being the purpose of Prof. Whitney, in case of a resumption of 

 the labors of the California Survey, to publish fuller descriptions, 

 illustrated by proper figures of each species. 



In addition to the descriptions, I have added a few notes on 

 previously known forms, in amplification or correction of the 

 original descriptions. 



ORTHOCERAS, Brug. 

 0. BLAKEI, Gabb.— Pal. Cal., vol. I., p. 19, pi. 3, fig. 1. 



This species is quite abundant at all of the Triassic localities 

 of Nevada. A fragment before us, showing the remains of two 

 chambers, has a diameter of 2-3 inches. Its entire absence of 

 all ornament renders it very difficult to describe in such a way 

 as to distinguish it from allied species. It is not unlike 0. 

 duhium of San Cassian, but it seems to be distinguished from 

 that species by the septa being more closely placed, in the 

 specimen just mentioned, the distance between each pair of the 

 three septal margins being -8 inch respectively. The septa also 

 appear to be somewhat more concave in the present than in the 

 European species. 



AMMONITES, Brug. 



A. Nevadanus, Gabb. — PI. 3, fig. 1, la. 



Shell large, discoidal, compressed, qjany whorled ; whorls in- 

 creasing very gradually in size, sections quadrate,' with slightly 

 convex sides ; dorsum tricarinate, the middle carina the highest ; 

 ■whorls in contact by their dorsal and ventral faces only. Sur- 

 face marked by numerous, equal transverse ribs, separated by 

 interspaces of about equal size ; on the dorso-lateral margin, 

 each rib bears a more or less distinct, small tubercle, after which 

 the rib bends suddenly forward and merges into the dorsal carina 

 of that side. Septum composed of a dorsal, a ventral, and a 

 single lateral lobe on each side. The dorsal and lateral lobes 

 are all broad, with the sides nearly parallel, and are divided at 

 the ends for about a third of their length, into two branches ; 

 the corresponding saddles are simple and bifurcate ; the ventral 

 lobe is unknown. 



