the teeth, are found
in a tolerable state of preservation; and these indications, fragmentary
as they are, give us the clue to the character of the most ancient
fishes. A large proportion of them were no doubt Ganoids; for they had
the same peculiar articulation of the vertebrae, the flexibility of the
neck, and the hard scales so characteristic of our Gar-Pikes.

There is another type of these ancient Vertebrates, which has also
its representatives among our modern fishes. These are the Sharks and
Skates, or, as the Greeks used to call them, the Selachians,--making a
very appropriate distinction between them and common fishes, on account
of the difference in the structure of the skeleton. In Selachians the
quality of the bones is granular, instead of fibrous, as in fishes; the
arches above and below the backbone are formed by flat plates, instead
of the spines so characteristic of all the fish proper; and the skull
consists of a solid box, instead of being built of overlapping pieces
like the true fish-skull. They differ also in their teeth, which,
instead of being implanted in the bone by a root, as in fishes, are
loosely set in the gum without any connection with the bone, and are
movable, being arranged in several rows one behind another, the back
rows moving forward to take the place of the front ones when the latter
are worn off. They are unlike the common fishes also in having the
backbone continued to the very end of the tail, which is cut in uneven
lobes, the upper lobe being the longer of the two, while the terminal
fin, so constant a feature in fishes, is wanting. The Selachians
resemble higher Vertebrate types not only in the small number of their
eggs, and in the closer connection of the young with the mother, but
also in their embryological development, which has many features in
common with that of birds and turtles. Of this group, also, we find
numerous remains in the ancient geological deposits; and though we have
not the means of distinguishing the species, we have ample evidence for
determining the type.

This combination of higher with lower features in the earlier organic
forms is very striking, and becomes still more significant when we find
that many of the later types recall the more ancient ones. I have called
these more comprehensive groups of former times, combining characters of
different classes, synthetic or prophetic types; and we might as fitly
give the name of retrospective types to ma