380 L. Lesquereux on the Coal Formations of N. America. 
the vegetable remains known in the Silurian and lower Devo- 
nian belong to species of fucoides or marine plants, mostly of 
small size, resembling some species of Fucus of our time. The 
Ss. 
At coal No. 1B, the second above the Millstone-grit, we have 
apparently the maximum of predominance of species of large 
size, especially of Lepidodendron. From this horizon upw sa 
as coal No. 12. 
As fast as these species of trees decrease in number, the ferns 
mostly of small size invade the coal-fields. They become pre 
dominant and show the greatest number of species at the base of 
the Mahoning sandstone. : 
This substitution of species is not the result of any perceptible 
change in their character or in their relative affinity. The rela- 
tion of Lepidodendron is to Lycopodites. Both genera appear Of 
at least disappear at the same time, and are replaced by typical 
forms, which have no analogy whatever with them. 
The distribution of the ferns in the coal-measures is equally 
contrary to the supposition of a change of species by successive 
it j ey together, in a kind 
of relation between contemporaneous species; but we dono 
see, either before or after any of them, a trace of an intermediate 
