necting
link-between the tortoise and the fish. I submitted some of my specimens
to Mr. Murchison, and they furnished him with additional data by which
to construct the calculations he was then making respecting fossils, and
they added a new and very singular link to the chain of existence in its
relation to human knowledge. Agassiz confirmed the conclusions of
Murchison in almost every particular, deciding at once that the creature
must have been a fish.

Next to the pterichthys of the Lower Old Red Sandstone I shall place its
contemporary the coccosteus of Agassiz--a fish which in some respects
must have resembled it. Both were covered with an armour of thickly
tubercled bony plates, and both furnished with a vertebrated tail. The
coccosteus seems to have been most abundant. Another of the families of
the ichthyolites of the Old Red Sandstone--the cephalaspis--seems
almost to constitute a connecting link between fishes and crustaceans.
In the present creation fishes are either osseous or cartilaginous, that
is, with bony skeletons, or with a framework of elastic,
semi-transparent animal matter, like the shark; and the ichthyolites of
the Old Red Sandstone unite these characteristics, resembling in some
respects the osseous and in others the cartilaginous tribes. Agassiz at
once confirmed my suspicion that the ichthyolites of the Old Red
Sandstone were intermediate. Though it required skill to determine the
place of the pterichthys and coccosteus there could be no mistaking the
osteolepis--it must have been a fish, and a handsome one, too. But while
its head resembled the heads of the bony fishes, its tail differed in no
respects from the tails of the cartilaginous ones. And so through the
discovery of extinct species the gaps between existing species have been
bridged.

_III.--Place-Fixing in the Dim Past_

The next step was to fix the exact place of the ichthyolites in the
geological scale, and this I was enabled to do by finding a large and
complete bed _in situ_. Its true place is a little more than a hundred
feet above the top, and not much more than a hundred yards above the
base of the great conglomerate.

The Old Red Sandstone in Scotland and in England has its lower, middle,
and upper groups--three distinct formations. As the pterichthys and
coccosteus are the characteristic ichthyolites of the Lower Old Red
formation, so the cephalaspis distinguishes the middle or coronstone
division of the system in E