CEPHALOPODA. 325 



sinus, in the margins of the large aperture. The length of the large aperture 

 is equal to three times the length of the sinus forming the small aperture. 



Air-chambers regular, having a depth of three mm. Septa smooth, and of 

 moderate concavity. Sutures straight and horizontal. 



Siphuncle small, moniliform near the ventral side ; sometimes exposed in 

 the process of weathering. 



Test and surface-markings unknown. 



The largest chamber of habitation observed has a length of twenty-five 

 mm., and a lateral diameter at the last septum of twenty-seven mm. A frag- 

 ment of the septate portion has a length of sixteen mm., and embraces six 

 air-chambers. 



This species is distinguished by its size, the curvature of the tube, and the 

 characters of the chamber of habitation. The amount of the constriction at 

 the aperture varies considerably in different individuals, and also the slope of 

 the dorsal and ventral sides. It is distinguished from G. beta in its greater 

 curvature and more slender form. A fragment of the chambered portion was 

 originally considered as belonging to Cyrtoceras, but the discovery of numer- 

 ous chambers of habitation of a species of Gomphoceras, conforming to the 

 requirements of the septate fragment, has induced its reference to the latter 

 genus The specimens from the Upper Helderberg limestones, at Clarence 

 Hollow, show slight variations from those found in the Schoharie grit, but the 

 differences are not of varietal or specific importance. 



Formation and localities. In the Schoharie grit, at Schoharie, and in the lime- 

 stones of the Upper Helderberg group, at Clarence Hollow, Erie county, N. Y. 



