22 A POPULAR EXPOSITION OF THE 
ig probably identical with the European species, O. regulare ; whilst O. tenu- 
filum may perhaps be referred to O. cochleatum (Schlotheium); O. bilineatwm to 
O. calamiteum (Munster); and Endoceras proteiforme to Schlotheim’s O. vagi- 
natum. Gonioceras anceps, on the other hand, is quite distinct from the Ortho- 
ceras anceps of De Koninek, and also from the earlier and doubtful O. anceps of 
Count Munster. An extended discussion of synonymes, or minute comparison 
of specific details, would be quite out of place, however, in an Essay of the 
present character. 
Cyrtoceras :—This genus includes the eurved orthoceratites with 
normal shell-aperture. The septa are simply concave, or slightly 
sinuated, and the siphuncle variable. Its forms, as at present known, 
may be arranged under two sub-genera, representing the first and 
third amongst the straight or true orthoceratites. The genus ranges 
from the lower Silurian into the Carboniferous formation. 
The first sub-genus, Cyrtoceras proper, has a gradually tapering 
and more or less slightly curved shell, with small siphuncle: the latter 
occupying a central or sub-central position, or 
lying along the larger curve of the shell. Fig. 
132 is a sketch of C. annulatum from the lower 
part of the Trenton group. 
In the second sub-genus, characterised by a 
large siphuncle as in the endoceratites, we may 
place the Piloceras of Salter. This form pre- 
sents short, thick, and slightly curved shells 
with large siphuncle. The latter often contains Fig. 132. 
a cone of calcareous matter, as in Hnxdoceras 
proteiforme. The type, as yet, is comparatively rare, but a species 
has been discovered in the Calciferous Sand Rock of the Mingan 
Islands, by Sir William Logan and Mr. Richardson. This is de- 
scribed by Mr. Billings in the Canadian Naturalist, Vol. V., p. 171. 
In making P2éoceras, however, merely a sub-genus of Cyrtoceras, as 
explained above, we follow our own views. 
Phragmoceras:—This genus, in form, is closely allied to Cyrto- 
ceras, and is also confined to Palzeozoic rocks. The shell is curved, 
and the septa simple or slightly sinuated; but the aperture of the 
shell is more or less strongly contracted. The siphuncle is variable: 
although in most species hitherto referred to Phragmoceras, it lies © 
along the shorter curve of the shell. In the Bohemian P. perversum 
of Barrande, and in the P. prematurum of Billings, it occupies, 
nevertheless, the convex side. Fig. 133 represents a fragment of the 
