CEPHALOPODA. 361 



tation. Tube regularly and gradually enlarging from the apex. Apical 

 angle of the lateral faces about 20°. Apex acute. 



Chamber of habitation large, gibbous, having a length greater than the 

 greatest transverse diameter. ^ The sides are convex, and slope gradually to 

 the aperture. Crenulated zone not well defined, but marked by shallow 

 furrows, of which there are three in the space of four mm., and are con- 

 tinued over the cast of the walls of the air-chambers. Aperture not fully 

 observed. 



Air-chambers regularly increasing in depth from the apex, varying from 

 one to six mm. in the length of seventy-five mm. Septa smooth, thin, 

 having a concavity equal to the depth of the air-chambers. Sutures hori- 

 zontal, curving slightly forward over the ventral side, and in the opposite 

 direction on the concave dorsal side. 



Siphuncle moniliform, ventral, submarginal ; having a diameter between 

 the septa of four mm., where the lateral diameter of the tube is thirty mm. ; 

 often exposed in the process of weathering. 



Test and surface-markings unknown. 



Internal mould essentially smooth, with the exception of the inconspicuous 

 furrows of the crenulations. Sutures slightly impressed. 



An individual nearly entire, embracing the chamber of habitation with 

 twenty-seven air-chambers, has a length of 110 mm., of which thirty- 

 five mm. pertain to the chamber of habitation. Its greatest ventro-dorsal 

 diameter is thirty mm. 



This species is distinguished by the curvature of the tube, the transverse 

 section, the apical angle, and the curvature of the septal sutures. 



Formation and localities. In the limestones of the Upper Helderberg group, at 

 Clarence Hollow, Erie county; Littleville, Livingston county, N. Y. ; and in 

 the same horizon in Ontario, Canada West. 



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