CEPHALOPODA. 
477 
Volutions six or more, in entire individuals, all exposed in the very wide, 
shallow umbilicus, a small proportion only (less than one-fifth) of the inner 
being overlapped by the outer ones, which are grooved upon their inner 
margins for the reception of the periphery of the inclosed volution; the 
groove has a depth of about two mm. where the dorso-ventral diameter of 
the embracing volution is twenty-one mm. Transverse section of the outer 
volutions trapezoidal (of the inner ones semi-elliptical), the sides nearly 
flat, the apex truncated, with the angles rounded; the base is concave in the 
middle, with the baso-lateral margins rounded. The greatest transverse 
diameter is equal to two-thirds of the dorso-ventral diameter, the difference 
in the proportions increasing in the outer volutions. The volutions enlarge 
very gradually; the measurements give seven and thirteen, and eleven 
and eighteen, or an increase of six and seven mm. in a single turn. In a 
larger individual the increase in half a volution is from nineteen to twenty- 
three mm., or equal to eight mm. in a single volution. 
Chamber of habitation unknown, all the specimens observed being incom¬ 
plete. Aperture unknown. Air-chambers numerous, slightly irregular in 
depth, and as a rule, gradually increasing toward the outer chamber. 
Septa strong, distinctly thickened and imbricating at their margins, and 
remarkable for the regularity of their curvature. From their origin they 
advance toward the aperture, in a nearly direct line, obliquely over the 
umbilical margin to a point on the lateral, face about one-fifth of its diameter; 
thence bending abruptly backward they describe an extremely abrupt curve, 
including a linguiform mucronate lobe, and return to a point in the centre of 
the volution greatly in advance of their first retral bending. From the 
summit of this curve or saddle on the centre of the face of the volution they 
describe a somewhat longer and scarcely wider retral and advancing curve, 
which in like manner limits a narrow linguiform and mucronate lobe, and 
from which the septum curves over the margin of the periphery, thence 
abruptly descending and turning acutely forward describes a narrow triangular 
mucronate lobe on the ventral side. This order gives an obtusely subtrian- 
gular saddle on the umbilical margin, a narrow lobe, and a very narrow and 
