CEPHALOPODA. 319 



Gomphoceras, Cyrtoceras, Gyroceras and Nautilus, including several minor 

 genera or subgenera. The distinctions between the genera, as now recog- 

 nized, do not furnish consistent or satisfactory results when applied to the 

 species here described. A generic definition, with illustrations of the species 

 here described and not figured, will be given in a future supplementary 

 notice. 



The comparative distribution of the species of this genus, in the formations, 

 is nearly parallel to the distribution of the species of Orthoceras, with the 

 exception of a greatly increased number of the former in the limestones of the 

 Upper Helderberg group, especially in its western extension. The species in 

 the Schoharie grit are in general comparatively slender. The Upper Helder- 

 berg limestones are characterized by several very large gibbous forms ; those 

 of the Hamilton, Portage and Chemung groups are marked by a large propor- 

 tion of short, ovoid forms, and the occurrence of several large fusiform species. 



In comparison with the species of Bohemia, as illustrated and described by 

 M. Barrande, in Syst. Sil. du centre de la Boheme, the American forms of the 

 genus offer a decided and constant difference. The large and small apertures 

 of the Bohemian species are comparatively smaller, and generally widely 

 separated, being connected by a narrow channel. The species described in 

 this volume are characterized by a transverse suboval, trilobate .aperture, which 

 is essentially one. The large aperture constitutes the two larger lobes, and 

 the confluent small aperture forms the smaller lobe. 



Several of the American forms here referred to the genus Gomphoceras pos- 

 sess features of the aperture intermediate between this genus and Mesoceras, 

 as constituted by M. B arranhe. The characteristics of the latter are mainly 

 baaed upon a single transverse, elliptical aperture, with an undefined sinus in 

 the stria; and margins, corresponding to the small aperture and ventral side of 

 the shell. In the majority of the species here described the large aperture is 

 transverse, and forms the principal feature of the entire aperture, showing a 

 regular gradation in the different forms, from a slight sinus or bend in the ventral 

 margin, to a distinct but confluent aperture, corresponding to the small aper- 

 ture. See G. oviforme, G. Conradi, G. clavatum, G. mitra, G. Fischeri and G. absens. 



