CEPHALOPODA. 347 



This species resembles G. oviforme, but is distinguished by its curved tube, 

 apparently larger aperture, the position of the plane of greatest transverse 

 section, and the rapid slope of the sides of the chamber of habitation to 

 the comparatively small aperture. The same differences distinguish the G. 

 raphanvs. 



Formation and locality. In the shales of the Hamilton group near Cazenovia, 

 Madison county, N. Y. 



GOMPHOCERAS RAPHANUS, n. Sp. 

 PLATE XCIV, FIGS. 2-5, 10. 



Shell small, straight, slender, exogastric. Transverse section subcircular or 

 very broadly oval. Longitudinal section fusiform. Plane of greatest trans- 

 verse section at the base of the chamber of habitation. Apical angle about 

 28°. Apex acute. 



Chamber of habitation small ; length less than the diameter of the tube at 

 the base. Sides convex, and sloping rapidly to the aperture. Crenulated 

 zone narrow, and marked by obscure furrows, which are carried over the 

 walls of the air-chambers. 



Aperture, as observed in a specimen much flattened from compression, 

 large, having a width equal to that of the grand chamber, or to twice the 

 length of the entire aperture. The minor aperture is marked by a small, 

 semicircular sinus from the ventral margin of the large aperture. 



Air-chambers regular, showing but slight variation from near the outer 

 chamber to the apex, and having a depth of from four to five mm.; the last 

 one or more, at the base of the chamber of habitation, usually shallower than 

 the preceding. 



Septa smooth, thin, slightly concave, the concavity not amounting to the 

 depth of one air-chamber. A small areola around the siphuncle indicates its 

 abrupt expansion in the cavities of the air-chambers. Sutures straight and 

 horizontal. 



Siphuncle near the ventral side, small, nummuloid, with a diameter at the 

 septa of less than one-half the diameter at the expanded portions. 



