joint, instead of by inverted cones, as in the ordinary fishes.  Their scales are also peculiar, being covered by enamel so hard, that, if struck with steel, they will emit sparks like flint.  It is on account of this peculiarity that the whole group has been called Ganoid.  Now, though we have not found as yet any complete specimens of Silurian fishes, their disconnected remains are scattered profusely in the early deposits.  The scales, parts of the backbone, parts of the skull, 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 many of the later groups,