CEPHALOPODA. 
301 
This genus is essentially a Silurian form in America and other countries 
where it has been recognized. All .the species here described are from the 
lowest member of the Devonian system, and have not been observed in the 
succeeding formations. One-half of the species named in the above historical 
sketch are from the horizon of the Niagara group; and it would seem probable 
that the genus here reached its maximum development. M. Barrande has 
indicated forty-five species from the Silurian system of Bohemia, among which 
are several groups of forms which are similar to a natural grouping of the 
species here described. Their specific relations will be noticed in the descrip¬ 
tions following. 
In the Schoharie grit the forms of Trochoceras seem to represent the genus 
Nautilus, which has not been observed to occur in that horizon. The Hamilton 
group has furnished a variety of species of the latter genus, but as yet no forms 
which can be referred to Trochoceras. 
9 
List of the species of Trochoceras described in this volume: 
Trochoceras, 
Clio, Hall . 
discoideum, “ 
Biton, “ 
eugenium, “ 
Orion, “ 
Barrandei , “ 
Pandion, “ 
obliquatum, “ 
expansum, “ 
• ) 
}> Annulated forms. 
> Smooth forms, making more than one volution. 
Smooth forms, making less than one volution. 
