CEPHALOPODA. 363 



Cyrtoceras densum, n. sp. 



PLATE LXXXV, FIGS. 17-19. 



Shell very slightly curved ; curvature greater toward the apex ; exogastric. 

 Transverse section broadly oval ; lateral diameter the longer. Tube regu- 

 larly and gradually enlarging. Apical angle about 12°. 



Air-chambers regular, numerous, gradually increasing in depth toward the 

 chamber of habitation, having a depth of four mm. where the tube has a 

 dinmeter of thirty mm., and of seven mm. where the tube measures fifty mm. 



Septa smooth, thin, with a concavity equal to an arc of about 102°, or 

 equal to the depth of the chambers. A small areola around the insertion of 

 the siphuncle marks its expansion in the cavities of the air-chambers. 

 Sutures straight and horizontal. 



Siphuncle large, nummuloid, near the ventral side, having a diameter 

 between the septa of nearly twice the depth of the air-chambers, or more 

 than one-fifth the diameter of the tube. 



Internal mould smooth, with the sutures but slightly impressed. 



One large fragment, with a lateral diameter at the smaller extremity of 

 forty-three mm., and embracing twelve air-chambers, has a length of seventy- 

 five mm. Smaller fragments have been observed, preserving these pro- 

 portions. 



This species is related to C. Hector, and may be distinguished by its rigid 

 suture-lines, which in that species are curved. The relations of these species 

 to the genus Orthoceras are not evident from the specimens examined, and 

 they cannot be grouped with any of the species of that genus here described. 

 The barge, excentric siphuncle and rapidly enlarging tube, showing a slight 

 curvature, more nearly assimulate them with species which have been consid- 

 ered as Cyrtoceras. 



Formation and locality. In the calcareous beds of the Hamilton group, at 

 Cumberland, Md. 



