dosteus of the
Ohio and St. Lawrence. But the arrangement, though it seemed at the time
one of the best and most natural possible, failed to meet any
corresponding arrangement in the course of geologic history. The place
assigned to the class of fishes as a whole corresponded to their place
in the Palaeontological scale;--- first of the vertebrate division in the
order of their appearance, they border, as in the "_Animal Kingdom_" of
the naturalist, on the invertebrate divisions. But it was not until the
new classification of Agassiz had ranged them after a different fashion
that the correspondence became complete in all its parts. First, he
erected the fishes that to an internal cartilaginous skeleton unite an
external armature of plates and points of bone, into his Placoid order;
next, gathering together a mere handful of individuals from among the
various orders and families over which they had been scattered,--the
sturgeons from among the cartilaginous fishes, and the lepidosteus and
polypterus from among the Clupia or herrings,--he erected into a small
ganoid order all the fishes that are covered, whatever the consistency
of their skeleton, by a continuous or nearly continuous armor of
enamelled bone, or by great bony plates that lock into each other at
their edges. Out of the remaining fishes,--those covered with scales of
a horny substance, and which now comprise nearly nine tenths of the
whole class,--he erected two orders more,--a Ctenoid order, consisting
of fishes whose scales, like those of the perch, are pectinated at
their lower edges like the teeth of a comb, and a Cycloid order,
composed of fishes whose scales, like those of the salmon, are defined
all around by a simple continuous margin; and no sooner was the division
effected than it was found to cast a singularly clear light on the early
history of the class. The earliest fishes--firstborn of their
family--seem to have been all placoids. The Silurian System has not yet
afforded trace of any other vertebral animal. With the Old Red Sandstone
the ganoids were ushered upon the scene in amazing abundance; and for
untold ages, comprising mayhap millions of years, the entire ichthyic
class consisted, so far as is yet known, of but these two orders.
During the times of the Old Red Sandstone, of the Carboniferous, of the
Permian, of the Triassic, and of the Oolitic Systems, all fishes, though
apparently as numerous individually as they are now, were comprised