172 GENESIS OF THE ARIETIDA. 
smooth, as in Arnioceras, on the first three whorls, the pile being acquired only 
on the last quarter of the third or first quarter of the fourth volution. They are 
only about twenty-five in number on the fourth whorl, and gradually decrease in 
number on subsequent whorls. 
The sutures on the second quarter of the sixth whorl show close affinity for 
Arn. semicostatum. This resemblance ceases in a great measure after the beginning 
of the fifth volution, when the pile assume an aspect similar to those of Cul, rari- 
costatum. ‘The resemblance is due to the fact that the pile remain undeveloped ; 
if they become more prominent, the shell would be more like semicostatum. The 
keel is also not so well developed as in semicostatum, and on this account resem- 
bles that of a species of Caloceras. The abdominal lobe at this time is slightly 
longer than the superior laterals, and these are about one half longer than the 
inferior laterals. The superior lateral saddles are broad, shallow, and deeply 
divided by only one marginal lobe. The inferior lateral saddles are tongue- 
shaped, and slightly deeper than the superior laterals. The lobes and saddles 
are serrated, and the first auxiliary saddle is very small. There are four speci- 
mens in the Museum of Stuttgardt which entirely confirm these observations. 
They are from Behla, and labelled Amm. kridion, Quenst. One is more com- 
pressed than the other, and closely approximates to Arn. semicostatum; in fact, 
the young shell is quite as smooth as the gibbous variety of that species, and 
with the exception of the abdomen, keel, and sutures it is identical with it. 
The original of Quenstedt’s description from Bebenhausen, the type in Sua- 
bia, bears out these remarks in every particular; but in his “‘ Ammoniten des 
Schwibischen Jura,” Figure 7 has channels and a form not identical with others 
of this species, all of which are similar to the young of Cor. kridion during the 
adult stage. 
Arnioceras? Nevadanum,! Hyarr. 
Amm. Nevadanus, GAs, Am. Journ. Conch, Philad., V., 1869, p. 6, pl. iii. fig. 1; IV. pl. xvi. 
This interesting form was found near Volcano, in Nevada, and probably 
belongs to the Lower Lias. The young as figured by Gabb is smooth, and 
the late stage at which the pila were introduced, their linear, straight aspect, 
and crowded arrangement throughout the young whorls, are unquestionably 
arnioceran. The sutures also, as figured in Volume IV. Plate XVI., have the 
characteristic outlines of this genus. Nevertheless, the older whorls have the 
form and proportions of Ver. Conybeari, and are also tuberculated, the abdomen 
being keeled and channelled. The abdomen has, however, much broader chan- 
nels than any specimens of Vermiceras yet observed. The character of this part 
is evidently not unlike that of the specimen in the Stuttgardt Museum described 
in note to page 70 as occurring in the Angulatus bed and yet having channels 
1 The species associated with this by Gabb, under the name of Amm. Colfaxi (Am. Journ. Conch., 
1869, V. p. 7, pl. iv. fig. 2, IV. pl. xvi.), and reported as found in the Lias on the western slope of the 
Sierra Nevada near Colfax, was in poor condition, and was consequently so badly represented in the figure 
as to be indeterminable. 
