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 it was a tuberculated 

 arnioceran form. 



Arnioceras Humboldti, Hyatt. 



Locality. — Humboldt County, Nevada. 



This species is closely allied to Am. 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 Am. 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 Am. Hartmanni, 

 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;- 

 ment represented in Figure 31 is 48.5 



mm., and the keel is a trifle more prominent than it appears in the drawing. 

 The pilse 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 corning 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. 



