CEPHALOPODA. 
405 
Chamber of habitation very large, in one specimen occupying more than 
half a volution; its full extent not known; its capacity is at least twice as 
great as all the chambered portion of the shell. Aperture elongate, semi¬ 
elliptical, narrowing toward the anterior margin, which is subacutely rounded. 
Base deeply indented by the preceding volution, and inferring from the form, 
it is subauriculate at the baso-lateral -angles. Air-chambers numerous, often 
somewhat irregular in their depth, those near the chamber of habitation 
sometimes shallower than those preceding. In a portion of the tube, which 
has a lateral diameter of about forty mm. at the smaller extremity, 
and of forty-six mm. at the base of the chamber of habitation, making 
about one-quarter of a volution, there are nine air-chambers, which vary 
in depth from six to ten mm., as measured from the summits of the lateral 
saddles, the shallowest being the last but one. In a smaller individual there 
are nine chambers in less than half a volution of the septate portion pre¬ 
ceding the commencement of the chamber of habitation. 
The septa are strong, especially in older shells; much thickened on their 
exterior margins, and strongly imbricating. In their course from their 
origin on the umbilical margin they curve more or less abruptly backward 
to a point from one-quarter to one-third the diameter of the volution, or 
sometimes even less, where they make an acute return, and curving forward 
over the lateral face of the volution, make a retral bend, which terminates at 
a point within one-fifth or one-sixth of the width of the volution from the 
peripheral margin, whence they make an acute turn forward, and pass over 
the margin of the periphery in an abrupt curve, and descending slightly, 
describe a narrow, acute lobe upon the centre of the ventrum. This course 
of the septa includes a narrow acute lobe, near the inner margin of the volu¬ 
tion, and thence describes a broad, obliquely semi-elliptical saddle; a second 
narrow, elongate acute lobe, near the outer margin, and a narrow obtuse 
saddle on the periphery, with one side extremely elongate, and the other 
very short. The septum describing the wide lateral saddle extends forward, 
in its advancing curve, to a distance equal to the greatest depth of nearly 
two air-chambers beyond its origin at the umbilical margin. In its retral 
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