8 AMERICAN JOURNAL 



bilobate ; superior lateral lobe long, narrow and irregularly 

 divided at the end into two branches, the lower of which is again 

 divided ; above these branches it has three unequal spurs on 

 each side ; lateral saddle divided into two branches ; below these 

 are four small lobes, much smaller than the superior lateral, but 

 with apparently the same general structure. 



Dimensions of a very much distorted specimen with eight 

 whorls. — Diameter, 4*5 in., true diameter perhaps about 3-o-4 

 in. ; height of body whorl, -9 in. 



Localities. — From the Jurassic (Liassic ?) slates of the west 

 slope of the Sierra Nevada. One specimen from the cut of the 

 Pacific Rail Road, " Station 2777, Sect. 53, one mile west of 

 the town of Colfax ;" another, very compressed, but recog- 

 nizable both from its style of volutions and remains of septum, 

 from Robinson's Ferry, on the Stanislaus River, given us by 

 Mr. Pomier. 



Both these specimens are much injured by distortion, and the 

 septum can be traced only with great difficulty. The dorsal 

 lobe is entirely obliterated, and the portion of the septum from 

 the inferior lateral to the umbilical suture is so imperfectly 

 preserved that I can only obtain it approximately. It is figured 

 in Jour. Conch. 1868, PI. 16, the upper of the two septa. The 

 matrix is, in both cases, partially metamorphosed, and in the Colfax 

 specimen the shell lies at riglit angles to the stratification of 

 the rock, to which fortunate circumstance we are indebted for 

 not having it pressed perfectly flat. In the other case, the shell 

 lying parallel with the slate, it is mashed down to less than a 

 tenth of an inch in thickness, and every rib is obliterated. 



The present species is most closely allied to A. Solaris, Phil,, 

 of the Lias, but it has more num.erous whorls ; Solaris is slightly 

 carinated, and the whorls seem to have been higher and nar- 

 rower. So far as we can decide from the material known, A. 

 CoJfaxii is rounded on the back. It has more lobes than Solaris^ 

 though of the same general style. The superior lateral in the 

 latter species is more distinctly trifurcate. 



A. BILLINGSIANUS, Gabb. — PI. 5, fig. 3. 



Palaeontology of Gal., vol. 1, p. 27, pi. 5, fig. 20. 



• "When I described this species I could obtain but a very in- 

 distinct trace of the septum. Since then I have had the good 

 fortune of finding a specimen showing the septum Avith unusual 

 clearness. It consists of a dorsal, two lateral, and three small 

 supplementary lobes. The dorsal lobe is narrow, bifurcate for 

 more than half its length, each branch consisting of a simple 



