GUELPH FAUNA IN THE STATE OF NEW YORK 95 



sentative of the genus Poterioceras, as denned by Hyatt, primitive in so far 

 as its aperture is not yet laterally contracted and still possesses a wide open, 

 roundly triangular outline ; also primitive in its less developed fusiform 

 shape, which, it appears, does not attain its most typical expression till Car- 

 bonic time. This genus, in its restricted sense, was placed by Hyatt 

 {Genera of the Fossil Cephalopoda)* under the term Acleistoceras, but 

 subsequently was regarded by him as a synonym of Poterioceras ; Acleisto- 

 ceras nobis includes brevicone fusiform bodies with partially contracted 

 living chamber. The aperture has a large ventral sinus and a dorsal 

 saddle, and is only slightly smaller in diameter than the living chamber, 

 while the outline is usually subtriangular. The siphuncle remains ventral, 

 and the form in section is an oval with the dorsum broader than the venter. 

 It appears from the frequent coincidence of shallow ultimate camerae 

 indicating gerontic conditions, with marked contraction of the aperture, 

 that the latter feature alone may indicate gerontic condition rather than 

 specific character. This fact would, in some measure, conform to the state- 

 ment made by Hyatt, that Oncoceras is a phylogerontic group, one of the 

 phylogerontic characters being the transverse contraction of the living 

 chamber during gerontic age. On the other hand, it was pointed out by 

 Clarke that the expansion of the shell during later growth and a sudden 

 contraction at the close of the swelling near the aperture is a character 

 appearing in the early genera Oncoceras, Clinoceras, Poterioceras and 

 Cyrtoceras (in the old sense), and that in the Devonic species of the ortho- 

 ceran genus Bactrites this expression of the shell characterizes the growth 

 stage directly succeeding the formation of the protoconch. It is in line 

 with the latter observation that, as Foord remarks, the inflation of Poterio- 

 ceras is much less conspicuous in the adult than in the young. This author 

 probably refers only to late species of Carbonic age. It would seem to us 

 that we have here a gerontic character indicating early decline of a series 

 of cephalopods taking place remarkably soon after the inception of the 

 cephalopod stock. In considering the small size and slight development 



'Boston Soc. Nat. Hist. Proc. 22: 277. 



