CEPHALOPODA. 269 



the internal mould in this species are very similar to 0. luxum. In its mode 

 of occurrence and condition of preservation, together with several specific 

 characters, it is very similar to that species. It is distinguished by its 

 greater apical angle and more regular organic deposit. 



Formation and locality. In the Upper Helderberg limestone at Stafford, N. Y. 



Orthocekas sirpus, n. sp. 



PLATE CXU, FIGS. 3, 4. 



Shell straight, slender, very gradually enlarging. Transverse section circular. 

 Apical angle about 4°. Apex unknown. 



Chamber of habitation cylindrical, with a gentle constriction near the 

 aperture. Air-chambers regular, large, having a depth of about seven mm. 

 where the tube has a diameter of nine mm. On the internal mould the 

 walls are smooth and flat. 



Septa and siphuncle not observed. Sutures straight and horizontal, but 

 slightly impressed on the cast of the interior. 



Surface marked by fine, rounded, slightly-curving, regular, transverse 

 striae of growth. 



One specimen preserving the chamber of habitation, and several of the 

 adjacent air-chambers, with traces of the surface ornamentation, has a 

 length of forty-five mm., with a diameter of nine mm. at the outer chamber. 

 Another individual in the same association, and retaining the test over the 

 entire tube, has a length of seventy-eight mm. 



This species somewhat resembles O. jaculum, but has a more attenuate tube 

 and distant septa. It is distinguished from O. stylus, of the Schoharie grit, by 

 the much greater depth of the air-chambers, and the gentle constriction of the 

 outer chamber. 



Formation and locality. From the Corniferous limestone at Columbus, Ohio. 



