CEPHALOPODA. 365 



From some indications on the specimen described, the curvature appears 

 to be exogastric, and the siphuncle ventral or on the convex side. 



Test not preserved. Tube ornamented with regular longitudinal furrows, 

 six in the space of six mm. where the tube has a diameter of twenty-four mm. 



The specimen figured, and the only one yet observed, has a length of 

 thirty mm., with diameters of twenty-six and thirteen mm. respectively at 

 the two extremities. 



This species is unlike any of the Devonian forms yet noticed, and it is not 

 sufficiently defined to admit of a satisfactory generic reference. 



Formation and locality. In the Goniatite limestone of the Marcellus shale, at 

 Manlius, N. Y. 



Cyrtoceras alternatum, n. sp. 



PLATK XI.YI, FIGS. 13, 13. 



Cyrtoceras undulatum, Il.wr,. Illustrations of Devonian Fossils: Cephalopoda, pi. 47, fig's. 9, 10. 1S76. 

 Not Gyrocerat undulatum, Vaxitxkm Geolog. Surv. N. Y. : Rep. Third Dist., pp. 139, 140, fig. 2. 1842. 



Shell slender, slightly curved, exogastric. Transverse section very broadly 

 oval or subcircular. Tube regularly enlarging from the apex to the aperture, 

 with the exception of the nodes and constrictions, which form the orna- 

 mentation of the surface. Apical angle about 8°. 



Chamber of habitation having a length more than one-third greater than 

 the lateral diameter of the tube at the last septum. Aperture apparently 

 entire, with a slight sinus in the margin on the ventral side. 



Air-chambers regular, gradually increasing in depth toward the chamber 

 of habitation, varying from four to six mm. in the length of seventy-five mm. 

 Septa smooth, moderately concave. Sutures straight and horizontal. 



Siphuncle small, cylindrical, close to the ventral side, exposed in the pro- 

 cess of weathering; having a diameter of 2.5 mm. where the tube measures 

 twenty-seven mm. 



Test strong, having a thickness of two mm. on the chamber of habitation. 

 The septate portion of the tube is ornamented with regular, rounded annula- 

 tions or undulations, occurring at about every second air-chamber. Each 

 annulation is marked by about twelve rounded nodes, and these are arranged 



