CEPHALOPODA. 429 
direction is about 104°. From the centre of the dorsal side of the volution 
the septa are directed obliquely forward to the dorso-lateral angle, where 
they are recurved, making a gentle inflexion over the side of the volution, 
and again advancing to the ventro-lateral angle, are more abruptly bent 
backward and curve to the centre of the ventrum. The septa in this manner 
describe a distinct angular saddle on the umbilical margin, a broad shallow 
lobe on the lateral face of the volution, a distinct angular saddle on the 
ventro-lateral margin, and a rounded lobe on the ventrum. The suture lines 
are distinctly impressed. 
Siphuncle ventral, near the surface, section circular, its diameter about 
one-seventh of the dorso-ventral diameter of the tube at the same point; 
slightly contracting between the septa, with its greatest diameter at its 
passage through the septa. 
Test strong in old shells, having a thickness of one mm. on the side of the 
volution, and becoming thicker at the margins, where it is produced into 
strong nodes on the ventro-lateral angles, and makes a sharp carina on the 
dorso-lateral angles. Surface cancellated bj' line undulating striae of growth, 
which are crossed by very fine interrupted, revolving striae. The tube is 
ornamented on the ventro-lateral angles with sharp nodes, of which there 
are two to every air-chamber. The concentric striae make a deep retral 
curve on the middle of each lateral face, but do not follow the curvature oi 
the septa; and in the same manner there is a retral curve on the ventrum, 
where they follow essentially the course of the septa. 
The internal cast is mostly smooth, with the exception of the broad 
suture lines and the nodes upon the angles of the periphery. The ventrum, 
along the air-chambers, is marked by a well-defined, elevated line or carina, 
which is frequently limited on each side by a shallow groove. 
The diameter of the disc, in large individuals, is 140 mm. The smallest 
specimen observed has a diameter half as great. 
This species is easily distinguished from any other by the non-re-entrant 
volutions, the transverse section and form of aperture, the septa, siphuncle, and 
surface-markings and ornamentation. In a comparison with N. (D.) inopinatus, 
