OP CONCHOLOGY. 7 



Dimensions. — Diameter of the largest specimen, with eight vo- 

 lutions, 6-5 inches ; section of outer whorl, vertical diameter, 1-2 

 in., transverse diam., -9 in. The inner volutions have the two 

 diameters about equal. 



Locality. — Two specimens from Volcano. Jurassic. Col- 

 lected by Mr. Clayton. 



A very remarkable peculiarity of one of the specimens before 

 me is represented on plate 16, Journal of Conch., 1868. The 

 figure of the end of a broken whorl shows the position of the 

 siphuncle to be in one of the lateral carin?e, the whole septum 

 being carried over with it. Tlie ventral lobe is obliquely oppo- 

 site, on the angle of the umbilicus, and the lateral lobes are 

 equally misplaced. The lateral lobe of one side is much larger 

 than the other, and is much better developed, and the dorsal 

 saddle of that side is nearly twice as wide as the corresponding 

 opposite saddle. The line d represents the position of the 

 median line of the dorsum; s, the position of the siphuncle, and 

 u u the umbilical angles. This is, in all probability, only an 

 individual distortion, but I am unfortunately unable, with the 

 material in my possession, to prove my opinion. 



As compared with known species, the present one differs from 

 A. bisulcatus, Brug., in having the whorls higher than wide, 

 while in that shell the reverse is the case ; in Nevadanus the 

 dorsal grooves occupy nearly the whole dorsum ; in bisulcatus 

 they take up less than a third of the surface, and in the latter 

 species the dorsal ribs are much more prominent. 



A. bisulcatus has a trifurcate superior lateral lobe, while in 

 Nevadanus it is bifurcate. 



A. jBonardi, d'Orb., has more numerous whorls, the ribs are 

 curved and the whorls are proportionally narrower ; it also has 

 nearly the same style of septum as A. bisulcatus, but the dorsal 

 saddle is tribolate. It differs from A. Conybeari, Sby., in the 

 whorls being less numerous and wider, the dorsum flatter, and 

 in the ribs being more distinctly bent as they approach the dor- 

 sal margin. Conybeari has lobes wide above, and narrow below, 

 and the dorsal saddle is tribolate. 



A. COLFAXII, Gabb. — PI. 4, fig. 2. 



Shell many whorled, flat, discoidal, whorls rounded on the 

 sides and dorsum, emarginated on the ventral face, increasing 

 very gradually in size. Surface ornamented by prominent 

 radiating ribs with broad concave interspaces; these ribs start 

 at the umbilical margin and cross the sides, straight or slightly 

 sinuous, and apparently become obsolete on the dorsum. Sep- 

 tum ; dorsal lobe unknown ; dorsal saddle broad and deeply 



