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 inm., 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 
