CEPHALOPODA. 311 



There is a close resemblance between this species and the succeeding 

 (0. Demus) ; but a marked difference is observed in the position of the siphuncle, 

 and the air-chambers in the latter are comparatively shallower. 



Formation and locality. In the calcareous beds of the Chemung group, near 

 Ithaca, N. Y. 



Orthoceras Demus, n. sp. 



PLATE XC. FIGS. I, 4, 5. 



Shell straight, regularly enlarging. Transverse section circular. Apical 

 angle 6°. Initial extremity unknown. 



Chamber of habitation not observed. Air-chambers regular, increasing in 

 frequency toward the apex, having a depth of from two to three mm.; 

 varying to this degree in the length of forty-five mm. 



Septa smooth, thin, having a concavity greater than the depth of the air* 

 chambers. Sutures straight and horizontal. 



Siphuncle small, excentric, and at the septa is distant from the nearest 

 point on the margin about one-third the diameter of the tube. The other 

 elements have not been observed. 



Test not preserved, surface-markings unknown. The internal mould is 

 essentially smooth. 



A chambered fragment, with a length of fifty-seven mm., and diameters 

 of twelve and eight mm. at the two extremities, shows twenty-eight air- 

 chambers. A smaller fragment of another example has twenty-three air- 

 chambers in the length of thirty-two mm. 



This species is associated, in some localities, with O. fulgidum ; and may be 

 distinguished by its excentric siphuncle and somewhat shallower air-chambers. 



Formation and localities. In the calcareous beds and shales of the Chemung 

 group, at Ithaca, and in the arenaceous shales at Philipsburgh, N. Y. 



