GEPHALOPODA. 
435 
their diameter, and gradually more in proportion as the outer volution 
becomes expanded toward the periphery, embracing about two-thirds the 
width of the preceding volution. Umbilicus large and open, exposing all the 
volutions of the spire; its sides in the young shells rounded, and in the older 
shells abrupt, leaving the margin obtusely angular, and descending almost 
rectangularly into the cavity. A transverse section presents the inner 
volution with a circular outline, the dorsal side of which is concave from 
embracing the preceding volution; the dorso-ventral diameter is about three- 
fourths as great as the transverse, varying in proportion at each successive 
volution. In the outer volutions the section presents an elongate, trapezoidal 
form, in which the dorso-ventral diameter is equal to once and a half to twice 
the basal diameter, which is measured from the greatest expansion at the 
margin of the umbilicus. The exterior or peripheral side is flattened in old 
specimens and measures about one-half, or less than one-half, the diameter 
across the base. The basal line is convex on each side, and concave in the 
centre for the reception of the embraced volution; the lateral faces are limited 
by gently curving lines, which become more nearly straight toward the 
aperture. The increase of width is nearly or quite double for each volution. 
Initial extremity small, globular, and closely surrounded by the succeeding 
volutions. 
Chamber of habitation extremely large, occupying fully two-thirds of the 
last volution, with capacity at least four times as great as all the air-chambers 
together. Aperture of the same form as a section of the last volution, and, 
judging from the best preserved specimens, it is slightly expanded on the 
lateral margins, and much expanded at the baso-lateral angles, becoming 
auriculate; the anterior margin is deeply sinuate, with a shallower sinus in 
the lateral margins. In imperfect specimens these features are indicated 
by the strise upon the surface. 
Septa deeply concave, and with some exceptions, regularly increasing in 
distance toward the outer chamber; in mature specimens they are ten or 
twelve mm. distant from each other, and in the extremely large specimens' 
more than fifteen mm. distant; sometimes several of the later septa are nearer 
