I 12 rAUEOXTOLOGV OF NEW YORK. 



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 abruptly 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 mm , the width of the volution at the last septum 

 i- seven mm., and the diameter of the siphuncle about one nun. 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 

 stria?, which at intervals are raised in fascicles, giving an undulated aspect, 

 which is often more conspicuous in young than in older shells. The stria) 

 become more conspicuous as they pass from the lateral faces over the borders 

 of the periphery. There are other fine, gently undulating, concentric stria?, 



