254 
PALAEONTOLOGY OF NEW YORK. 
retained by the casts of the interior, and show that it was marked by fine, 
curving, transverse striae. 
The internal mould is smooth, and the chamber walls essentially flat. 
A specimen 130 mm. long, and preserving about ninety-five mm. of the 
chamber of habitation, is scarcely appreciably diminished towards the apex, 
the greatest actual diameter being in the middle of the length. Another 
fragment of ninety-five mm. in length, of which thirty-five mm. pertain to 
the chamber of habitation, shows scarcely a diminution in the diameter. 
The first of these specimens has a diameter of fifteen mm., and the second 
of twelve mm. 
This species is remarkable for its slender, cylindrical form, resembling 0. 
exile of the Hamilton group, but is distinguished by its smaller size, greater 
attenuation, and the absence of any constriction of the chamber of habitation. 
From 0. tetricum it differs in its very small apical angle, the much greater 
frequency of the septa, and the central or slightly excentric siplmncle. 
In its vertical distribution this species is confined to the Schoharie grit, but 
it is one of a small group of forms continuing through the Upper Helderberg 
limestones, and the Hamilton group. 
Formation and localities. In the Schoharie grit at Schoharie, and the Helder¬ 
berg range. 
Obthoceras medium, n. sp. 
PLATE LXX1X, PIGS. 11, 12. 
Shell straight, gradually expanding. Transverse section circular. Apical angle 
about 5°. Initial point unknown. 
Chamber of habitation not observed. Air-chambers irregular in depth, 
increasing toward the grand chamber, varying from five to ten mm. in the 
length of forty mm. In the cast, the ventral side of the chamber walls 
is ornamented by a elongate-conical carina, extending from the anterior 
portion of each chamber about two-thirds of its length. 
Septa thin, smooth. Sutures horizontal, straight. 
