436 PAUEONTOLOOT OF NEW YORK. 



together than those preceding them. The concavity of the septa is. greater 

 than the depth of the air-chambers. The depth of the lobe is equal to the 

 depth of two air-chambers. The margins of the septa advancing from their 

 origin define a low axial saddle upon the umbilical wgle, and thence gently 

 curving backward include on each lateral face a single broad, flattened lobe 

 which occupies the entire width of the volution. The summit of the saddle, 

 occupying the angle formed by the lateral and peripheral faces, is rounded, 

 curving abruptly upon the ventral side, and more gently on the lateral face, 

 and becoming much more abrupt, and even subacute, in the older shells. 

 The ventral lobe is very narrow and deep, with the sides, in young indi- 

 viduals, nearly parallel, and extending for half the depth of the adjacent 

 air-chamber; the walls at its lower extremity sometimes appear to be con- 

 tinuous and coincident with the walls of the siphuncle. In older specimens 

 the sides are less nearly parallel and the form is triangular. The substance 

 of the septa is strong, and thickened at the margins, which are imbricating 

 toward the aperture. 



The siphuncle is well defined at the septa, close to the ventral side, and 

 distinctly circular in section. In a specimen where the lateral diameter of 

 the volution is forty-three mm., and its largest transverse diameter thirty- 

 three mm., the siphuncle has a diameter of three mm., and in a smaller 

 inclosed volution, which has a transverse diameter of about fourteen mm., 

 the siphuncle is more than 1.5 mm. in diameter. 



The test has a thickness of about one to two mm. on the outer chamber of 

 older individuals, and is about half as thick in the smaller ones. It is 

 always thicker toward the umbilicus, where it is sometimes three mm. The 

 surface is marked by strong, lamellose, curving striaa ; which in the young 

 shells are crowded into fascicles, rising into ridges, and crowned by one or 

 two stronger stria? at intervals of a millimetre, and increasing to two or 

 three mm., when they become less prominent, and finally subdued in the 

 general surface. These curving annulations are sometimes indicated, on the 

 cast of the chamber of habitation of the young shell, by low undulations, as 

 shown in figs. 6 and G of plate 69 ; but they maintain a distinctive feature 



