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 Ortiioceras, 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. da 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. Barrande. The characteristics of the latter are mainly 
based upon a single transverse, elliptical aperture, with an undefined sinus in 
the striae 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. 
