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 striae 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 striae of growth, and in a 

 few examples by gentle undulations, which indicate the course of the stronger 

 fascicles of the external striae. 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 ten 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 ventrolateral 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- 



