on Devonian fossils of New York. 49 
straight, and with thick shell; they are ornamented with strize 
or with oblique rings. This genus is limited to the Devonian 
formation. 
The Hyoliths are distinguished from all the other genera of 
American Pteropods by their geological extent; they appear 
in the Cambrian (Potsdam) and are found above the De- 
vonian and in the Carboniferous limestone. It is remarkable 
to see this genus diminish in the Middle Silurian and even dis- 
appear in the Upper Silurian, which is so rich in fossils and 
in other Pteropods, then reappear with six new forms in the 
Devonian. On the contrary, it is in the second and third Si- 
lurian faunas that this genus attains its full development in 
Kurope. There are nevertheless resemblances between the 
forms of the two continents: the Hyolithes aclis (Hall) of the 
Hamilton, resembles the Hyolithes discors of Barrande; the H. 
erica. Among the curved forms there are more relations 
he genus Orthoceras appeared in America in the Cambrian 
(Calciferous sandstone); the sub-genus Endoceras did not 
pass beyond the second fauna; the Orthoceras proper are 
found as far as the Permian. There are two principal horizons 
of Orthoceras in the Devonian formation; the Schoharie grit, 
reduc 
100 meters in the western part. The Schoharie grit passes 
'nsensibly into the Upper Helderberg limestone, and it is 
Am. Jour, oe Serizs, Vou. XXI, No. 121.—Jan,, 1881. 
