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. 
Orthoceras sirpus, n. sp. 
PLATE cxn, 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 
apertuje. 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. 
