284 
PALAEONTOLOGY OF NEW YORK. 
Septa smooth, thin, having a concavity equal to an arc of about 125°. 
Sutures straight and horizontal, but little impressed on the internal mould. 
Sipliuncle small, subcentral, only observed at the septa, where its diameter 
is one mm., the tube having a diameter of fifteen mm. at the same point. 
Test thin, usually showing a thickness of .2 mm.; sometimes it is con¬ 
siderably stronger over the anterior portion of the chamber of habitation. 
Surface marked by very fine, irregular, lamellose striae of growth. Some of 
the specimens show longitudinal striae, and more rarely very fine, filiform, 
longitudinal markings. 
The internal mould is essentially smooth, and the specimens from the 
softer shales often present a black and polished appearance. 
The individuals of this species never reach a large size. The nearly 
entire examples have a length of from 85 to 130 mm. 
This species is distinguished from 0. exile and 0. emaceratum by the position 
of the siphuncle, and much shallower air-chambers. It more nearly resembles 
0. constrictum; but the absence of the broad constriction removes it from that 
species. 0. tenere, from the same horizon at the Falls of the Ohio, closely 
resembles this species, but has a larger apical angle, and more regular, rounded, 
transverse strise. 
This is evidently the species referred to by Mr. Vanuxem as the “slender 
orthocera.” It is very common in the softer shales of both the lower divisions 
of the group, and is chiefly confined to the central portions of the State. 
Formation and localities. In the Marcellus shales at Bloomfield, and in the 
Hamilton shales at Delphi, Pratt’s Falls, and numerous other localities in the 
central portions of the State. 
