CEPHALOPODA. 335 



transverse furrows, of which there are four in the space of fourteen mm. 

 The crenulations are retained over the internal mould of the chamber walls. 



Aperture large, sub-oval, having a width equal to that of the grand 

 chamber. The small aperture is formed by a slight bend in the margins of 

 the large aperture. Ventro-dorsal diameter of the latter about two-thirds 

 that of its lateral diameter. 



Air-chambers regularly increasing in depth from the apex toward the 

 chamber of habitation, varying in depth from five to nine mm. in the length 

 of seventy-five mm. The last two chambers somewhat shallower than the 

 preceding ones. Septa smooth, thin, deeply concave ; their concavity greater 

 than the depth of two air-chambers. Sutures curved forward over the lateral 

 faces of the tube, but not oblique to the axis in their general direction. 



Siphuncle small, moniliform, with a diameter in the cavities of the air- 

 chambers of nearly four mm., where the tube has a ventro- dorsal diameter of 

 thirty -eight mm.; distant from the ventral side about one-fourth the ventro- 

 dorsal diameter. 



Test and surface-markings unknown. The septate portion of the tube in 

 the cast is marked by several obscure, transverse undulations of the shell, 

 with a variable frequency and prominence. 



The internal mould preserves the furrows of the crenulated band, con- 

 tinued over the walls of the air-chambers. Sutures but little impressed. 



The specimen described, consisting of the chamber of habitation and fifteen 

 attached air-chambers, has a length of 180 mm., and its greatest lateral diam- 

 eter is nearly eighty mm. 



This species is distinguished from the other species in the Upper Helderberg 

 limestones by the position of the point of greatest gibbosity, apical angle, the 

 size and characters of the chamber of habitation, and the position and dimen- 

 sions of the siphuncle. It is considerably larger than the species described 

 from the Schoharie grit. A slight resemblance is found with G. lunatum, of the 

 Hamilton group; but that species has a marked arcuate tube, with a larger and 

 submarginal siphuncle. G. potens, from the Chemung group, is similar in the 



