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. acrdus 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, ventricose. 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 striae, 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 



