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. eugenium and C. czmulum 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 Nercm, Hall Descriptions of New Species of Fossils, p. 39 1861. 



" Fifteenth Rep. N. V. 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. 



