CEPHALOPODA. 
373 
Internal mould marked by low annulations, which correspond in frequency 
to the transverse folds of the test, and showing lamellose, concentric lines 
from the external striae of growth. 
One fragment, comprising about one-fourth of one volution, has a length 
of 180 mm., with diameters of thirty and eleven mm. respectively at the two 
extremities. 
This species is distinguished from C. eugeniam and C. amulum by the greater 
curvature of the tube, larger apical angle, and the more slender plicate expan¬ 
sions of the test. In its ornamentation it more nearly resembles G. Nereus, 
but in that species the tube clearly makes more than an entire volution. The 
specimens preserved in the softer limestones of the West retain their normal 
form, and show the characters here mentioned in a very satisfactory degree. 
Formation and localities. In the limestones of the Upper Helderberg group; 
at Schoharie, Caledonia, Leroy, Falkirk, N. Y.; and at Cayuga, Ontario, Can. 
Gyroceras Nereus. 
PLATE LXI, FIGS. 4-6. 
Gyroceras Nereus, Hall. Descriptions of New Species of Fossils, p. 39 1861. 
“ “ “ Fifteenth Rep. N. Y. State Cab. Nat. Hist., p. 67, pi. 9, fig-. 4. 1862. 
“ “ “ Illustrations of Devonian Fossils: Cephalopoda, pi. 49, figs. 1-3. 1876. 
Shell subdiscoidal, very openly coiled; spiral rapidly enlarging, making about 
one volution and a half. Transvere section subcircular. Tube regularly and 
rapidly enlarging. Apical angle in a compressed specimen 14°. 
Chamber of habitation not observed. Some obscure, longitudinal furrows 
over the internal cast indicate the character of the crenulations. Aperture 
entire, with a small, semicircular notch on the ventral side, equal to the sinus 
of the ornaments on the test. 
Air-chambers regular, having a depth of about five mm. on the outer 
volution. 
Siphuncle ventral; not fully observed. 
