78 PALEOZOIC FISHES OF NORTH AMERICA. 
were ushered in by a shallowing of the sea and a spreading of sheets of 
sandstone and conglomerate—the Millstone grit—over a large portion of 
the bottom of the interior basin. These fragmental strata were deposited 
by violent action of shore waves and strong currents, which in many places 
deeply eroded or tore up the old sea bottom, and left in its place thick beds 
of gravel and cross-stratified sand. In some localities, as in northern Ohio, 
the conglomerate contains great numbers of imperfectly rounded fragments 
of the cherty layers of the Mountain limestone; showing that this forma- 
tion once existed there and that it has been extensively worn away. 
During the deposition of the Coal Measures there were great changes 
of level; land, shore, and deep-water conditions alternating repeatedly in 
the same locality. Gradually a series of ridges and troughs were produced 
parallel with the old axis of the Blue Ridge, and in the irregularly subsid- 
ing troughs the coal strata—old peat beds—were formed at the water level, 
and successively buried by subsidence and the deposition over them of 
gravel, sand, clay, and marl; now conglomerate sandstone, shale, and lime- 
stone. 
Naturally the life of the Carboniferous age was distributed according 
to these differences of physical conditions. On the land grew forests com- 
posed of the characteristic vegetation of the age; in the lakes and rivers 
were mollusks and fishes adapted to their places of residence; while in the 
sea different groups of marine animals inhabited the shallows and the deeper 
basins. 
Owing to the long continuance of the Carboniferous age and the diver- 
sity of habitat offered them, the forms of invertebrate and vertebrate life 
buried in the strata are exceedingly diversified. Among the fishes—which 
are the objects of the present investigation—the number of genera and spe- 
cies already described is large, and yet we have evidence in the constant 
additions making to the list that our knowledge of the fish fauna is yet very 
imperfect. We have learned enough, however, to be warranted in saying, 
(1) that fishes were much more numerous and varied in this than in the 
preceding age, though that is called the age of fishes; (2) that the Elasmo- 
branchs were far more numerous and powerful than in the Devonian sea; 
(3) that the Ganoids and Placoderms had been largely superseded by the 
