FISHES OF THE CARBONIFEROUS SYSTEM. 79 
Sharks, and were, for the most part, driven to the shores and bodies of fresh 
water. 
The influences which produced the great revolution in the fish fauna 
between the Devonian and Carboniferous ages are unknown to us, and will 
perhaps always remain so. If the interval was a long one, we can imagine 
that the changes took place in the sea basins, whose sediments are beyond 
our reach, and in the natural way of spontaneous variation, and the survival 
of the fittest; but here as elsewhere in geological history we must wonder at 
the absence of transitional forms. During the deposition of the great mass 
of the Mountain limestone, where it is at least 1,200 feet in thickness, there 
was a change of fauna that has led geologists to divide the mass into four 
groups: the Burlington, Keokuk, St. Louis, and Chester beds: which, with 
much in common, have each certain fossils peculiar to itself. But we do 
not find what we should naturally expect, that these differences are the 
results of gradual modifications of the earlier into the later species. Con- 
necting links are wanting, and the changes of fauna seem to have been pro- 
duced by importation rather than modification. Where, as in Kentucky, 
this great mass of organic débris has been produced by the growth, death, 
and disintegration of successive generations inhabiting the sea at the same 
place, it would seem inevitable that we should find abundant evidence of 
the transformation of the older species into the later. But little or nothing 
of this kind has been discovered. Certain species run through the entire 
mass with little perceptible change, while others are added as though by 
importation. This is a problem which will undoubtedly occupy paleontol- 
ogists for ages to come, and with more abundant material its solution may 
be made clear to all. At present it is beyond our reach. 
One striking peculiarity of the Elasmobranch fishes of the Carbonife- 
rous age is, that so many of them were provided witli defensive spines. It 
it true that this feature was not confined to the Carboniferous Sharks, for 
the spines of Machaeracanthus in the Devonian, and those of Hybodus in the 
Jurassic, are perhaps as formidable as any others; but the defensive spines 
found in the Carboniferous rocks outnumber ten to one those of all the 
other geological systems, and they surpass in very much greater proportion 
anything we find in the living fauna. Sharks are very numerous and formi- 
