CEPHALOPODA. 
451 
to the chamber of habitation, which is more rapidly expanding. Umbilicus 
small, rounded and very neatly defined, with the margins subangular, and 
the sides nearly vertical, the outer volution embracing almost the entire 
width of the inner volution. Transverse section in young shells broadly 
semi-elliptical or semicircular, with the basal or inner margin deeply indented 
by the preceding volution; the outer volutions and section of chamber of 
habitation semi-elliptical, with the vertical much greater than the transverse 
diameter; the sides curving gently, the apex rounded, and the base deeply 
indented; the baso-lateral angles are slightly auriculate. The enlargement 
in a single volution is from eleven to twenty-one mm. — the last measure¬ 
ment being over the first half of the grand chamber; and in one-half a turn 
of the last septate volution the increase is from eleven to fourteen. 
Chamber of habitation moderately large, gradually expanding and some¬ 
what ventricose, occupying apparently the greater part of one volution. 
The aperture is somewhat broadly semi-elliptical, with the sides gently 
curved, the anterior margin abruptly rounded and deeply sinuate. The base 
is deeply indented by the preceding volution, and the lateral angles slightly 
auriculate. Air-chambers irregular, gradually expanding from the umbilical 
margin, with the greatest depth usually upon the periphery; their depth 
on the lateral face varies from 1.5 to 4 mm. in less than half a volution on 
the same specimen, being equal to the greatest depth on the peripheral 
margins. 
Septa thin, unequally distant at their origin, radiating from the umbilical 
margin, and curving gently forward to the middle of the lateral face of the 
volution, thence gently backward to a point about two-thirds the distance 
from the centre of the umbilicus to the periphery, where they suddenly bend 
backward, almost at right angles with their previous direction, to a point 
below that of their origin, and in a line nearly parallel to the periphery; 
following and partially producing the inner margin of a shallow, revolving 
groove upon the surface; thence bending gently forward to near the centre 
of the periphery where they turn abruptly backward, defining the short, 
narrow, ventral lobe, which penetrates a little more than one-third the depth 
