414 PAL.EO.STOLOtlY OF SEW YORK. 



There is likewise sometimes, a distinct longitudinal carina, marking the sur- 

 face of the filling of the air-chambers along the centre, on the convex ventral 

 side; and the nine feature is shown in the inner lamina) of the shell. In 

 the same position the cast sometimes preserves a gentle sinus on the ventral 

 side of the chamber of habitation. 



This species, in the larger individuals, has a diameter of about ninety 

 mm., with a transverse diameter of the tube, near the aperture, of sixty- 

 five mm. 



This form is distinguished from N. liratus by the absence of the strong, 

 revolving plications, the greater curvature of its volutions and the position of 

 the siphuncle. From N. cornulum it differs in the proportionally greater trans- 

 verse diameter, and the simple contiguity of the volutions. It has a wide 

 vertical and horizontal distribution in the shales of the Hamilton group, 

 occurring also in the Goniatite limestone. It becomes more common in the 

 coarse, arenaceous shales of the lower part of the group in Schoharie, Otsego 

 and Madison counties, especially near Cazenovia. It occurs in the higher, 

 semi-calcareous shales of the group in the central and western part of the State. 

 It also occurs near Cumberland, Md., and the adjacent part of Virginia. 



Formations and localities. In the Goniatite limestone, near Manlius ; in the 

 Hamilton group, at Schoharie; Cazenovia; on the shores of Cayuga and Can- 

 andaigua lakes; near Geneseo, Avon and York, in Livingston county, and 

 other localities in Central and Western New York; and at Cumberland, Md. 



Nautilus cornulum. 



PLATE LX, FIGS. 5, 6. 

 XautUui cornulum. Hall. Illustrations of Devonian Fossils : Cephalopoda, pi CO, figs. 5, 6. 



Shell subdiscoid, ovoid, gibbous. Volutions about two or more, slightly 

 re-entrant, exposing nearly their entire width. Umbilicus profound. Trans- 

 verse section subcircular, with a distinct sinus on the concave dorsal side, 

 where it embraces the adjacent volution. Tube regularly and rapi lly 

 enlarging. Apical angle about 16°. 



