on Devonian fossils of New York. 51 
comprises at present, according to Mr. Hall, many species 
which should be removed from the genus. The Gyroceras, 
which are Cyrtoceras with volutions more enrolled and sepa- 
rated, present so many relations, in the position of the siphon 
and the ornamentation, with the American Cyrtoceras, that one 
cannot really consider them as distinct generic types: but that 
they may be reunited. us the series of Cyrtoceras alternatum, 
eugenium, citum, is parallel with the series Gyroceras Nereus, 
trivolve, laciniosum, Matheri, paucinodum, undulatum ; we fol- 
low all the changes in the two series from a like initial form to 
a shell several times enrolled. A second group, comprising 
more massive forms, shows the same relations between the series 
of the Cyrtoceras Jason and the Gyroceras cyclops. The generic 
distinction of all these forms, based upon the degree of curva- 
ture of the shells, is entirely artificial, and neglects the essen- 
tial characters which unite all these shells in the same group. 
If it is easy to recognize a Cyrtoceras in the Silurian period, it 
becomes more and more difficult to do it with precision in the 
Devonian, in proportion as the Gyroceras become developed. 
The general law is nevertheless that the Gyroceras succeed the 
vyrtoceras in time. These forms are represented by six species 
in the Schoharie grit, twelve in the Upper Helderberg, six in the 
Hamilton and one in the Chemung. 
The genus Trochoceras, established by MM. Hall and Bar- 
rande for the Gyroceras enrolled in helix form, is essentiall 
Silurian in America as in Europe. It has attained its greatest 
development in America at the epoch of the Niagara. M. 
Barrande has described forty-five species in the Silurian of 
ohemia. In the Devonian of America, this genus is limited 
In the Devonian, the Nautilus are abundant, in place of the 
Orthoceras which we have seen so predominant in the Silu- 
Perhaps in the Upper Helderberg of Ohio, first appeared in 
