116 GEOLOGICAL SURVEY OF THE TERRITORIES. 
internal whorls, even before this period, are almost hidden. This is, 
therefore, similar to those forms among Aimonites, which I have so 
often described as accelerated types; those which display in the earlier 
periods of growth and development, in quick succession, characteristics 
which come out in slower* succession in other species. Like many of 
those forms, also, a kind of premature degeneration appears, even betore 
the animal can be said to have reached its adult condition. Thus at the 
diameter of an inch and a half, or even less sometimes, the sides of the 
whorls no longer increase by growth with the same rapidity as in the 
young. The amount of involution, consequently, is not maintained at 
the same rate, and the sides of the internal whorls become more exposed, 
until in some old specimens they are only about half covered up. 
“Thick folds or ribs are present in some old specimens, and a tend- 
ency to form nodes. In some large specimens the ribs cross the abdo- 
men, and in one fragment a curious effect is produced by the retention 
of the contrictions formed by the transient mouths of the shell. These 
make depressions with swellings between, which give the abdomen a 
scalloped appearance, amply sufficient to found a new genus upon if 
economically used. There may be two rows of slight nodes on either 
side of the abdomen, giving this shell a slight resemblance to Trachy- 
ceras in some cases. 
‘The forward part of the living-chamber seems to be more rounded 
or gibbous than the after part, which has the square abdomen, even in old 
specimens, though I think that i in extreme old age the whole shell would 
exhibit a round abdomen and more gibbous sides. These peculiarities 
and its flattened abdomen might readily mislead an observer to identify 
this species with Amm. semipartitus von Buch, but a glance at the septa 
would satisfy any one that they are distinct.”—(A. H.) 
MEEKOCERAS GRACILITATIS Var. 
Among the examples of I. gracilitatis, which were obtained at local- 
ity No. 1, is one which shows a considerable modification in the char- 
acter of the septa. These appear to be in all respects like those of the 
typical for ms, except as regar ds the inner lateral cell and the auxiliary 
series. The inner lateral cell is smaller than in the typical forms, and 
its inner border is not as abruptly defined from the auxiliary series. 
This series occupies a wider space, and consists of more numerous ser- 
rations than in the typical forms, appearing, indeed, to consist of a 
finely serrated, nearly straight suture. The umbilical shoulder is also 
less abrupt than in the typical forms. In all other respects this speci- 
men appears to conform to the typical specimens of the species, and for 
this reason a separate specific name is not proposed. 
Genus ARCESTES Suess. 
ARCESTES? CIRRATUS White. 
Arcestes? cirratus White, 1879, Bull. U. S. Geol. Sur. Terr., vol. v, p. 116. 
In his notes upon the Cephalopods of this article Professor Hyatt re- 
fers this form provisionally to the genus Arcestes with the following re- 
marks: 
* “This expression, of course, is relative; applying not to the absolute amount of 
time occupied in the growth, but to the age "at which the characteristics appear.” 
