442 
FA L2E0NT0L0G V OF NEW YORE. 
and, abruptly curving backward, forms a deep sinus on the periphery. Air- 
chambers very shallow near the axis of the shell, continuing nearly parallel, 
and then becoming rapidly expanded toward the periphery, gradually 
increasing in depth with the increasing size of the volutions. 
The septa, on each side, are closely arranged on the inner half of the 
volution, and first curve gently forward, and thence abruptl} T recurving on 
the middle of the lateral face, continue with a more gentle retral bend to the 
periphery, where they are more than three or four times as distant as at 
their origin. In their passage, they describe a broad and deep lateral lobe, 
which occupies the outer half of the volution; with a very shallow and 
scarcely marked saddle upon the peripheral border, and a very short and 
narrow, ventral lobe. In young shells the sides of the ventral lobe are nearly 
parallel, and extend about one-third of the distance across the adjacent air- 
chamber. The width of the saddle is equal to the width of the lobe, and 
the depth of the lobe is equal to one and a half or two air-chambers. The 
sutures are distinctly marked upon the exterior of the cast by the thickened 
margins of the septa, which are imbricating toward the outer chamber, and 
leave a strong groove when weathered. 
Siphuncle very small, close to the ventral side, and nearly coincident with 
the walls of the peripheral lobe. In one specimen, having its greatest lateral 
diameter of twenty-eight inm., the width of the volution at the last septum 
is seven mm., and the diameter of the siphuncle about one mm. on the convex 
side of the septum. In a specimen exposing the siphuncle between several 
of the septa, its form is evidently subconical, narrower at the concave face, 
and wider at the convex face of the septum. 
The test is very thin, in young specimens being not more than .2 mm., 
and in the older ones not more than .5 mm. The older shells are always 
extremely compressed. The surface is marked by fine, closely arranged 
striae, which at intervals are raised in fascicles, giving an undulated aspect, 
which is often more conspicuous in young than in older shells. The striae 
become more conspicuous as they pass from the lateral faces over the borders 
of the periphery. There are other fine, gently undulating, concentric striae, 
