CEPHALOPODA. 
443 
which are confined to a small area around the umbilicus, giving a delicately 
ornamented surface. Some of the specimens show a pitted or punctated 
surface, of similar character to that observed on the shell of the recent 
Nautilus beyond the covering of the mantle. The principal stride make an 
abrupt, retral bend upon the ventro-lateral margins, and describe a deep 
sinus upon the periphery. 
The internal cast, in a large proportion of the specimens, is essentially 
smooth, being marked only by impressions of the strire of growth, and in a 
few examples by gentle undulations, which indicate tin? course of the stronger 
fascicles of the external stria3. The marks of the undulating concentric striae 
around the umbilicus are rarely preserved in .the casts of the interior, but 
the punctate marking is more frequently seen. Small individuals of this 
species have a lateral diameter of teu mm., with a transverse diameter of 
six mm. The largest specimen measured, which is much compressed in the 
softer shales, has a greatest diameter of ninety-five mm. The prevailing 
forms are illustrated on plate 71. The natural rotundity of the young and 
medium sized specimens is illustrated in figures 5, 6, 8 and 9 of the same 
plate. 
This species differs conspicuously from the young of G. Vanuxemi by its closed 
umbilicus, and the usual absence of undulations or annulations upon the shell 
surface or upon the casts. It is very closely allied to G. uniangularis, Conrad ; 
but the original of that species is less rotund, and the septa are more distant at 
their origin, and on the inner part of the volution ; the lateral lobe is narrower, 
and the ventro-lateral saddle is much longer; while the dorsal lobe has similar 
proportions, the septa being comparatively more distant on the periphery. This 
species is less rotund than G. bicostatus, which it resembles in some of its 
features; but in that species the dorsal lobe and adjacent saddle are propor¬ 
tionally longer, and the lateral lobe narrower, while the septa do not make so 
abrupt a retral turn on the lateral face. In its vertical and horizontal distri¬ 
bution, this species has a greater range than any other of the group. It begins 
its existence in the Goniatite limestone of the Marcellus shale, where the young 
and smaller forms are of common occurrence. It likewise occurs in the Mar- 
