CEPHALOPODA. 285 



Orthoceras tenere, n. sp. 



PLATE CXI1I, FIG. 1.1. 



Shell small, straight, rapidly enlarging. Transverse section subcircular. Api- 

 cal angle about 10°. Initial point unknown. 



Chamber of habitation not denned. Air-chambers regular, having a depth 

 of one mm. In the cast the walls are smooth and flat. 



Septa having a concavity nearly equal in depth to the depth of the air- 

 chambers. Sutures straight and horizontal, but little impressed on the cast 

 of the interior. Siphuncle not observed. Surface marked by fine, slightly 

 curving, rounded, concentric striae of growth. 



The specimen on which the characters of this species are based has a 

 length of fifty mm., and a diameter of thirteen mm. at the larger extremity. 



This species is distinguished from 0. subulatum by its greater apical angle and 

 rounded, transverse strise ; and from 0. textum, with which it is associated, by 

 the surface ornamentation. 



As in the succeeding species, this form occurs in a completely silicified con- 

 dition, the internal characters being obliterated by the deposit of calcedony on 

 the interior of the tube, while the external form and markings are in a very 

 perfect state of preservation. 



Formation and locality. In the cherty beds, above the Corniferous limestone, 

 at the Falls of the Ohio, near Louisville, Ky., which are referred to the age of 

 the Hamilton group. 



Orthoceras textum, n. sp. 



PLATE CXIII, FIG 14. 



Shell small, straight, rapidly enlarging. Transverse section subcircular. Api- 

 cal angle about 11°. Initial extremity unknown. 



Chamber of habitation and the air-chambers not defined in the specimen. 



Septa not determined. The siphuncle lies close to the interior surface. of 

 the test, and the septa are not visible. This position may be owing to the 



