CEPHALOPODA. 
437 
over the chamber of habitation in the largest individuals, where the distance 
may be two, three, or four mm., or even more; as indicated on the specimen, 
pi. 67, toward the periphery. These striae, on the lateral faces, make a 
curvature very nearly corresponding to the curvature of the septa; and in the 
young shells they make an abrupt retral curve over the low saddle, thence 
to the middle of the periphery, where they are recurved; forming a deep 
sinus upon the ventrum. The place of the saddle is indicated by a double 
revolving groove, margined on each side by a low carina, and separated by 
a stronger similar one, As the shell enlarges, the lateral of these two 
grooves first becomes obsolete, and finally also the other, leaving a simple 
obtuse angle along the line of the saddle, over which the striae in their retral 
curve are more conspicuous. The form of the sinus of the aperture is indi¬ 
cated by the direction of the striae on the ventrum. A single individual of 
the species shows a smaller sinus on each side of the centre, and thence a 
broad, shallow sinuosity reaching to the baso-lateral angles of the aperture. 
The internal cast, in the young specimens, often preserves the double 
ventro-lateral, revolving grooves, or frequently a single groove; and also the 
curving annular ridges upon the lateral faces. These are sometimes very 
conspicuous, gradually becoming obsolete on the chamber of habitation, as the 
shell increases in size. In the advancing growth of the shell the sides of the 
outer volution gradually become flattened, and often a little concave toward 
the outer margin, along a line parallel to the periphery. This feature is 
more conspicuous in old shells, and is especially marked in the specimen on 
plate 68. With these exceptions the cast of the interior is smooth. The 
suture lines are strongly impressed, and the septa are imbricating, leaving 
the posterior margin of the air-chamber curved or sloping, while the anterior 
edge is angular. A slender median ridge or carina has been observed along 
the centre of the ventrum, upon a partially exfoliated specimen, marking the 
inner or nacreous layer of the shell. 
The individuals of this species present a great range in size and proportions. 
The smaller specimens do not exceed thirty mm. in their greatest dimensions, 
with the dorso-ventral and lateral diameters of their volutions about as ten 
