. 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 filling 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 

 po ssessed by O. Tantalus. 



Formation and locality. This species is known to me only in the limestone of 

 the Upper Helderberg formation, at Delaware, Ohio. 



