452 
PALEONTOLOGY OF NEW YORK. 
of tlie adjacent air-chamber. In this manner the septa describe a wide, 
shallow saddle, which is abruptly truncate on the outer side, and occupies 
more than half the width of the volution; and thence includes a lateral lobe 
with a straight and nearly vertical inner margin, the base being in a 
direction with the origin of the preceding septum, and terminating on the 
periphery, leaving a narrow ventral saddle between it and the ventral lobe. 
The suture lines are strongly marked, and over the inner half of the lateral 
face of the volution, there is a scarcely perceptible imbrication toward the 
aperture; on the outer lobe and saddle there is a strong imbrication of the 
margins of the septa. The sipliuncle is small and close to the ventral side, 
but its other characters have not been determined. 
The test is thin. The surface, toward the umbilicus, is marked by some¬ 
what feeble striae, which become more conspicuous on the middle of the 
lateral face, and decidedly strong and elevated toward the periphery, which 
is limited on each side by a prominent, revolving carina, having on its inner 
side a narrow depression, which is more strongly marked in the cast. 
Crossing this carina, which is sometimes crenulate or nodulose, the striae 
bend abruptly backward, becoming stronger on the periphery. The sinus is 
not fully determined, but is ver}^ deep and apparently angular or subangular 
in the bottom. 
The interior cast, or the exfoliated shell, is usually more or less marked by 
the impressions of the exterior striae, which appear as low, undefined undu¬ 
lations. These markings are sometimes preserved where the interior cast is 
of iron pyrites. The lateral face, toward the periphery, is marked by a 
shallow, well-defined groove, which is limited externally by a sharp carinate 
line corresponding to the exterior carina. The centre of the periphery is 
marked by a narrow carina, which is perceptible in the inner shell-laminae. 
Many of the smaller specimens have a diameter not exceeding ten mm., 
while the larger ones are more than forty-five mm. in diameter. The speci¬ 
men figure 8, plate 72, is represented in its natural proportions; figure 9 is 
enlarged to two diameters. 
