FOURTH, OR CORONICERAN BRANCH. 173 
with entire lateral ridges, which occupy nearly the whole breadth of the abdomen. 
Gabb’s figures are a little at variance with his description on this point, and may 
have been taken from different whorls; one in Volume V. has narrow channels, 
while the section in Volume IV. Plate XVI. has broad channels. The section, 
description, and sutures show lateral distortion. If it were not for this and the 
possible errors of the figures, we should be positive that iti was a tuberculated 
arnioceran form. 
Arnioceras Humboldti, Hyarr. 
Locality. — Humboldt County, Nevada. 
This species is closely allied to Arn. tardecrescens, from which, however, it differs 
in the sutures and proportions of the whorls, those of the latter being broader in 
proportion to their breadth. This species also probably reached a larger average 
size and had a thicker shell. It differs 
from Arn. Bodleyi in the same charac- 
ters, and the sides are less divergent 
outwardly than in that species, and not 
so flat. The great thickness of the 
shell reminds one of Arn. Hartmann, 
but the whorls increase faster by 
growth in the abdomino-dorsal diame- 
ter, and it has a smaller number of 
whorls at the same age. The figures 
are close to the natural size, and give 
accurately the proportions of the fos- 
sil. The actual diameter of the frag- 1 Fie. 32. 
ment represented in Figure 31 is 48.5 
mm., and the keel is a trifle more prominent than it appears in the drawing. 
The pile were much abraded and are truthfully portrayed in this figure, but 
when entire they possessed the usual sharpness of the genus Arnioceras. 
The transverse section represented in Figure 32 is in part a restoration, and 
has been slightly reduced in the cut. The extreme breadth of the side and keel 
is 19.5 mm. The sutures are given with sufficient accuracy in Fig. 33, but the 
median lobe which divides the superior lateral saddles has been overlooked, and 
the abdominal lobe has not been indicated. The latter has the usual arnioceran 
proportions, being somewhat shorter than the superior laterals. The marginal 
lobe dividing the superior lateral saddles is unusually broad, and in fact the 
broad massive aspect of the two lateral saddles is a marked characteristic of 
these sutures. 
In the collection of the Mining Bureau at San Francisco is a specimen 
of Arnioceras, which, judging from a hasty sketch, resembles this species. It 
was labelled as coming from Inyo County, California, but we were not able to 
verify the locality. It is, however, quite certain that species of the Lower 
Lias have been found in the West in the regions occupied by the exposures of 
the Jura. 
