CEPHALOPODA. 
413 
as all the chambered portion of the shell. Gently constricted near the 
aperture. Aperture straight, entire, opening horizontally and transverse to 
the axis of the shell. Margin slightly curving toward the apex on both the 
dorsal and ventro-lateral sides. Air-chambers regular, gradually increasing 
in depth toward the grand chamber. In a fragment of the chambered por¬ 
tion of the tube, measuring about sixty mm. on the curve of the convex side, 
and having a width of thirty-six mm. in the middle of its length, there are 
nine air-chambers. 
Septa thin, regular, having a concavity of about 110°, and equaling the 
depth of one and a half air-chambers; closely arranged on the axial margin, 
and distant six to ten mm. on the ventral side. Sutures distinctly marked, 
and often deeply impressed from the solution or weathering of the septal 
margins. 
Siphuncle near the concave dorsal side, and about one-fourth or one-third 
of the dorso-ventral diameter from 'the surface. Its elements between the 
septa not observed. 
Test thin, having a thickness of about one mm. on the dorsal side, 
and from .5 to .75 mm. on the convex ventral side. Surface marked by 
verv line lines of growth, and by regular, more distant, transverse striae. 
These are crossed by regular, sharply elevated, revolving striae, of which 
there are about four in the space of ten mm. near the aperture, and between 
these there are liner, irregular striae. The stronger revolving striae are not 
unfrequently very prominent, and assume the character of gentle car illa¬ 
tions on the concave side; but they are more subdued upon the convex 
side. In examples where the shell is partially exfoliated, the inner layer 
preserves the longitudinal, and to some extent the transverse striae, in a very 
subdued condition. 
The internal cast is essentially smooth, with the sutures distinctly 
impressed, and the substance of the filling strongly imbricating. Sometimes 
there are thin films of the macerated test remaining attached, which par¬ 
tially preserve the surface-markings; and the cast often retains distinct 
impressions of the revolving striae of the inner, nacreous layer of the shell. 
