CEPHALOPODA. 
237 
mm., the last air-chamber being shallower than any of the preceding. The 
depth of curvature of the arc measuring the concavity of the septa is about 
one-fourth the diameter of the tube at the same point; and the arc of the 
circle subtends an angle of about 95°. 
Siphuncle slightly excentric, being nearer to the convexo-ventral side of 
the shell, small at its passage through the septa; but a longitudinal section 
has given no satisfactory evidence of its character, or of its existence in the 
interseptal spaces. 
Test entirely unknown, and no evidence of ornamentation visible upon the 
cast of the interior. Internal cast of the outer chamber apparently smooth, 
the septate portion showing a moderate convexity of the tilling of each air- 
chamber, which may, however, be in part due to the mode of weathering. 
The specimen figured, and the only one positively identified, has a length 
of about 240 mm., with a diameter at the larger extremity of fifty-eight 
mm., and at the smaller extremity of thirty-five mm. The chamber of habi¬ 
tation measures 100 mm., and the septate portion, on the exterior, measures 
130 mm., and embraces twenty air-chambers. 
This species, from cursory examination, was originally noticed as a variety 
of 0. Pelops, the slight curvature being regarded as accidental. A study of the 
specimen proves it to be a very distinct species. The curvature is apparently 
normal, and the extension of the septa on the concave side offers farther 
evidence of this fact. In the dimensions of the siphuncle at the septa, and the 
distance and curvature of the septa, it resembles the 0. Tantalus , as shown in 
pi. 35, figs. 8-10, and pi. 35 A, fig. 7. In its external characters it might not 
readily be distinguished from that species, but the curvature and obliquity of 
the septa, and the excentricity of the siphuncle, are characteristic features. 
Besides, it has not the peculiar organic deposit on the septa and siphuncle 
possessed by O. Tantalus. 
Formation and locality. This species is known to me only in the limestone of 
the Upper Helderberg formation, at Delaware, Ohio. 
