CEPHALOPODA. 
471 
preserved for the extent of a quarter of a volution, its rate of increase is 
continuous with the preceding portion of the shell, being apparently a little 
more expanded on the umbilical margin, and carrying about four transverse 
constrictions on the inner half of the volution. The aperture, inferring from 
the form of the volution, as far as preserved, has been semi-elliptical, with a 
length greater than the transverse diameter at the base, which is invaded by 
the preceding volution to more than one-third of its length. The baso- 
lateral angles have been somewhat expanding and auriculate. The apertural 
sinus is undetermined. Air-chambers numerous, often quite regular, and 
having a depth on the middle of the lateral face equal to one-fourth the 
ventro-dorsal diameter of the volution; frequently quite irregular in depth, 
becoming shallower toward the aperture. The change from the deeper to 
the shallower air-chambers is sometimes abrupt; the latter, having about 
half the depth of those preceding, continue with great regularity to the base 
of the chamber of habitation. 
Septa thin, with the margins thickened and imbricating; turning back¬ 
ward from their axis, they are acutely recurved on the umbilical angle, and 
thence bending gently forward, make an advance equal to the greatest depth 
of two air-chambers, as measured on the lateral face of the volution; and * 
thence abruptly recurving, extend to a point on the face of the volution 
about one-third of its width from the peripheral margin, where they bend 
acutely forward to a point a little in advance of the previously described for¬ 
ward arch, and are thence abruptly curved over the margin of the periphery, 
and backward, almost in a line parallel with the direction of the volution, to 
a depth equal or greater than the previous central curve; and thence twice 
recurving in a narrow space, define the ventral lobe. By this course the 
septa define a narrow triangular lobe on the umbilical margin; a wide, mod¬ 
erately elevated saddle, the summit of which lies a little within the centre of 
the lateral face of the volution; a deep triangular lobe, with an acute base, 
the point of which lies about one-third of the width of the volution from the 
peripheral margin; and a narrower, more elevated saddle, the summit of 
which lies upon the curve of the periphery. The ventral lobe is tridentate 
