CEPHALOPODA. 289 
The internal mould of the chambers is smooth, with the sutures but little 
impressed. 
One fragment, comprising a portion of the outer chamber and twenty-one 
of the attached air-chambers, has a length of eighty-five mm. A septate 
fragment, retaining nearly its normal form, measures fifty-three mm., with 
diameters of seventeen and eleven mm. respectively, at the two extremities. 
The chamber of habitation has a length of seventy mm. in one individual, 
with a diameter of about fifteen mm., and is apparently not entire. 
This species is distinguished from 0. subulatum by its constricted and much 
more developed chamber of habitation. In the depth of the air-chambers and 
the position of the siphuncle the two species are very similar. It is farther 
removed from 0. exile by the depth of the air-chambers and its central 
siphuncle. The constriction is also generally broader and nearer the middle of 
the chamber of habitation, sometimes becoming nearly as prominent a feature 
as represented in Vanuxem’s original figure of the species. 0. emaceratum may 
be readily distinguished by the much greater depth of the air-chambers. 
Mr. Conrad* gave this name to an annulated form, which has not since been 
recognized. The absence of an illustration and the want of sufficient definition 
in his description renders it undesirable to change the present recognized 
species of Yanuxem. 
Formation and localities. In the Hamilton group, in the counties of Ontario 
and Madison, N. Y.; and at Cumberland, Md. 
* Mr. Conrad’s description is as follows : 
Orthoceras constrictum .—Shell smooth, tapering gradually, with a few remote transverse furrows. 
Locality, town of Madison, Madison county. (Geolog. Surv. N. Y.: Pal. Dep., Annual Report, p. 117. 1838.) 
37 
