38G PALAEONTOLOGY OF NEW YORK. 



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

 grand chamber, varying from three to seven mm. in the length of twenty- 

 eight air-chambers. Septa smooth, moderately concave, with an areola 

 around the insertion of the siphuncle. Sutures straight and at right angles 

 to the spiral axis. 



Siphuncle near the ventral side, moniliform; not satisfactorily observed. 



Test and surface-markings unknown. Internal mould essentially smooth, 

 with the sutures moderately impressed. 



The specimen -figured on pi. 100 has a diameter, measured from the aper- 

 ture across the disc, of 180 mm., and the greatest diameter of the tube is 

 fifty-two mm. 



This species is distinguished from G. spinosum by the absence of revolving 

 rows of spines or nodes, and a more gradually enlarging tube. It differs from 

 G. Eryx in its smaller volutions and more openly coiled spiral. 



Formation and localities. In the Schoharie grit, Schoharie .and Albany 

 counties, N. Y. 



Gyroceras Eryx. 



PLATES LVIII, FIG. 1 ; CIII, FIG. 3. 



Gyroceras Eryx, Hall. Descriptions of New Species of Fossils, etc., p. 39. 1861. 



Fifteenth Rep. N Y. State Cab. Nat. Hist., p. 07. 1S02. 



'• " " Illustrations of Devonian Fossils j Cephalopoda, pi. 48, fig. 1. 1876. 



Shbu. large, discoid, coiled, making about two nearly contiguous volutions. 

 Transverse section broadly oval ; ventro-dorsal diameter the longer. Tube 

 regularly enlarging from the apex to the aperture. Apical angle about 10°. 



Chamber of habitation large. Aperture not observed. 



Air-chambers numerous, regular, having, on the outer volution, a depth of 

 fifteen mm., measured on the convexo-ventral side, and of six mm. on the 

 concavo-dorsal side. Sutures straight and at right angles to the spiral axis 

 of the tube. 



Siphuncle ventral, near the chamber walls, nummuloid, expanding to a 

 diameter equal to the depth of the air-chambers. 



