112 PALAEONTOLOGY. 



merely digitate), to the fourth, which is very small, and nearly or quite 

 simple; first sinus between the siphonal and first lateral lobe smaller, but 

 of the same form as the latter, and merely obtusely serrated, while the suc- 

 ceeding sinuses decrease rapidly in size inward, and become more nearly 

 simple; siphonal lobe shorter than the first lateral, but of about the same 

 bi-eadth, with a short, spreading, digitate, terminal, lateral branch on each 

 side, and a few small lateral serratures along the lateral margins. 



Greatest breadth of a specimen apparently, retaining most of the outer 

 volution, 2.34 inches; convexity of same, about 0.55 inch. 



In fonn, this shell agrees pretty nearly with some varieties of G. Blahci, 

 Gabb (sp.); but it differs in being more compressed, particularl 3^ toward the 

 periphery, which is thus made narrower, and in its more deeply embracing 

 volutions, and consequent narrower umbilicus. Its costte are also, appar- 

 ently at all ages, excepting, perhaps, in the very young specimens, much 

 more obscurely defined. So far as the details of the lobes and intennediate 

 sinuses of its septa can be made out from the specimens, they seem to agree 

 exactly with those of G. BUikci. I had selected for it the name rotelliformh, 

 and written out a full description, with remarks on its points of difference 

 from, and agreement with, G. Blakei, but was not fully satisfied whether it 

 sliould be included pi-ovisionally as a marked variety of that species, or sep- 

 arated entirely as a distinct species. As Professor Hyatt confirms the latter 

 conclusion, I have decided to place it apart as a different species. Professor 

 Hyatt sent the following note respecting its relations to G. Blakei: 



"This seems a distinct species from G. Blalcel. The latter develops 

 from the young, in which the pilse are distinct, and the abdomen invariably 

 keeled at one stage; whereas similar pilaj and a keel are only occasionally 

 and faintly shown in the shell under consideration. The young are more 

 distinctly pileated than the adults, and are somewhat like the young of G. 

 Blakei at certain stages of growth. A close comparison, however, shows 

 them to be flatter, and nearly tlie whole of each whorl is concealed b}- the 

 involution of the shell, whereas G. Blakei has a more open umbilicus at the 

 same age. The same remarks a))ply equally well to the figure of G. Blakei 

 in the Palaeontology of California. 



"The general development and characteristics of the septa of this species. 



