CEPHALOPODA. 397 



Siphuncle moniliform, close to the convex ventral side, having a diameter 

 at the septa of five mm., where the tube measures about thirty mm. 



Test and surface-markings unknown. The internal mould is nearly 

 smooth, preserving some traces of transverse lines of growth. Sutures mod- 

 erately impressed. 



Numerous specimens have an average diameter of 100 mm., measured 

 across the disc, with a greatest diameter of the tube of forty mm. 



This species is distinguished from T. Orion by its less gibbous form, less 

 twisted and curved chamber of habitation, and more oblique septa. 



Formation and localities. In the Schoharie grit, Schoharie, and in the Held- 

 erberg mountains. 



Trochoceras Orion. 



PLATE LVIII, FIG. 2. 

 Ci/rtoceras Orion, Ham,. Illustrations of Devonian Fossils: Cephalopoda, pi. 48, fig. 2. 1876. 



Shell large, subdiscoidal, making more than one volution. Spire very depressed, 

 dextral. Umbilicus abrupt, profound, having a diameter less than the trans- 

 verse diameter of the grand chamber. Transverse section broadly oval; 

 transverse diameter the longer, and oblique to the axis of the spiral. Tube 

 rapidly enlarging to the middle of the chamber of habitation, and thence 

 more rapidly contracting to the aperture. * 



Chamber of habitation large, subovoid, length twice the ventro-dorsal 

 diameter, (sharply curved at the base, and near the aperture directed outward 

 from the spiral, tangent to the curve. It occupies about one-third of the 

 outer volution, and has a capacity of more than twice the volume of the air- 

 chambers. Aperture same as transverse section, with a distinct sinus in the 

 ventral margin on the convex side of the tube. 



Air-chambers regular, having a depth of eleven mm. on the convex side of 

 the tube, near the grand chamber. Septa smooth, moderately concave. 

 Sutures oblique, inclining toward the aperture on the upper, lateral face, and 

 toward the apex on the lower or umbilical side of the tube. 



