CEPHALOPODA. 
417 
This species may be distinguished "from N. cornulum by its more re-entrant 
volutions, the form of the transverse section, the more distant septa, and their 
curvature on the dorsal side. From N. bucinum it differs in its re-entrant volu¬ 
tions and deeper air-chambers. From N. acraus it may be distinguished by 
the same characters and by the surface-markings. 
Formation and locality. In the shales of the Hamilton group, near Cumber¬ 
land, Maryland. 
Nautilus acr^eus, n. sp. 
PLATE CIX, FIGS. 3-5. 
Shell small, obliquely subovoid, ventrieose. Volutions not exceeding two, 
contiguous or slightly re-entrant. Umbilicus very deep. Transverse section 
elliptical, with a gentle sinus on the concave dorsal side. The dorso-ventral 
and transverse diameters are about as two to three, the difference increasing 
in the grand chamber. Tube rapidly enlarging. In a somewhat flattened 
specimen, the apical angle is about 20°. 
Chamber of habitation large, its volume much greater than all the septate 
portion of the shell; its length twice the lateral diameter at the base. 
Aperture nearly straight, not oblique to the axis of the shell. Air-chambers 
of moderate depth ; not satisfactorily determined. 
Septa having a very moderate degree of concavity. Siphuncle small, sub¬ 
central, being a little above the centre on the dorsal side. 
Test thin. Surface marked by fine, irregular-lines of growth, and by fine, 
sharp, regular, and closely arranged revolving stride, about twelve in the 
space of four mm. on the convex side, and a little more distant on the 
concave side, of the outer volution. 
The internal cast preserves impressions of the transverse and revolving 
striae. The outer chamber of a small specimen has a length, as measured on 
the convex side, of a little more than forty mm., while its greatest diameter 
at the base is eighteen mm., and at the aperture, where it has been more 
compressed, thirty mm. A larger individual, somewhat more compressed, 
has a diameter at the base of the grand chamber, of thirty mm. 
53 
