CEPHALOPODA. 241 



Siphuncle central or subcentral, small, the diameter being about two mm. 

 where the tube has a diameter of thirty-five mm. The character of the 

 siphuncle in the interseptal spaces is unknown. 



Cast of the interior essentially smooth. The ventral side is indicated by 

 a longitudinal raised line or carina on the cast of the septate portion. Test 

 and surface-markings unknown. 



The incomplete individual, fig. 2, pi. 76, has a length of 220 mm., and a 

 diameter, at the base of the chamber of habitation, of fifty-five mm. 



The accompanying figures offer an explanation of the appearance, described 

 above, as produced by weathering. Figure 1 is a portion of a longitudinal 



section of the tube of a perfect specimen, 

 showing the epidermal layer, e, and the 

 nacreous layer, n. Portions of the septa 

 n«.«. are represented, continuing from s to z. 



C, C, are air-chambers filled with infiltrated matter, or material similar to the 

 surrounding rock. Figure 2, a section of a weathered specimen, with the 

 consequent removal of .all exposed shelly matter, z, z, are the zones, or narrow, 

 shallow furrows, as represented in figure 2, plate 76. 



This form is distinguished from 0. fludum by the distance and rigid appear- 

 ance of the septa, and their produced margins, and by the keel along the ventral 

 side. It also resembles 0. Tantalus, but the septa are not so frequent or oblique. 

 Formation and locality. This species is known only from the Schoharie grit, 

 in the vicinity of Schoharie, N. Y. 



Orthoceras Tantalus, n. sp. 



PLATES XXXV, FIGS. 8-10 ; XXXV A, FIGS. 7, 10. 



Shell large, straight, regularly expanding from the apex. Transverse section 

 circular. Apical angle 9° to 10° 30'. 



Chamber of habitation large; but its proportions are not fully deter- 

 mined, owing to the imperfection of the specimens. Aperture unknown. 

 Air-chambers numerous, regularly increasing in depth to the chamber of 

 31 



