CEPHALOPODA. 
381 
This species is very nearly related to G. undulatum, but may be distinguished 
by its more closely coiled volutions, triangular section, broad, flat, ventral 
side, and the more regular and distant nodes along the ventro-lateral angles of 
the tube. The general form of the shell, with the ornaments of the test, is 
similar to Hercoceras mirum , Barrande, from the Silurian of Bohemia; but the 
apertures in the two species are very unlike. 
Formation and localities. In the Upper Helderberg limestones at Cherry 
Valley, and Schoharie, N. Y. 
Cyrtoceras Jason. 
PLATE L, FIGS. 1, 2; AND SUPPLEMENT. 
Cyrtocwas Jason, Hall. Descriptions of New Species of Fossils, etc., p. 43. 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 71. 1862. 
Gyroceras “ “ Illustrations of Devonian Fossils: Cephalopoda, pi. 52, fig. 2. 1873. 
Shell large, robust, curved, making more than one-fourth of one volution. 
Transverse section subcircular, somewhat hexagonal, losing the angular 
character toward the apex of the tube. Tube rapidly enlarging. Apical 
angle about 15°. 
Chamber of habitation large, with a length greater than the diameter of 
the tube at the aperture. Crenulations preserved on the internal mould as 
numerous, shallow, longitudinal furrows. Aperture oblique, opening out¬ 
ward from the spiral axis of the tube. 
Air-chambers regular, having a depth of from five to ten mm.— a variation 
which is shown in different specimens. Septa smooth, with a concavity 
equal to the depth of the air-chambers. Sutures straight, and at right angles 
to the spiral axis of the tube. 
Siphuncle small, ventral, near the surface, sometimes exposed in the 
process of weathering. 
The test has a thickness of .75 mm. Surface marked by irregular, lamellose 
lines of growth. At intervals of about each alternate septum the lamellose 
lines of growth are projected into sharp, transverse, foliate expansions, 
inclining toward the aperture. Along the obtuse angles of the tube these 
