CEPHALOPODA. 



391 



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 

 Bpecies 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. 



List of the species of Trochoceras described in this volume: 

 Barrande. 



Annulated forma. 



Smooth forms, making more than one volution. 



r Smooth forms, making less than one volution. 



