many of these are more allied to the teeth of worms than of any other animals.[36] [Illustration: FIG. 103.—Lower Carboniferous Conodont. Magnified.—After Newberry.] If these older Conodonts were really teeth of fishes, they carry the introduction of these nearly as far back as that of the Mollusks and Crustaceans. If they were not, then the earliest known representatives of this class belong to a much later age, that of the Silurian. Here we have undoubted remains of fishes belonging to two of the higher orders of the class; and in the succeeding Devonian these became multiplied and extended exceedingly. Besides the inferior tribes already referred to, the modern seas and rivers present four leading types of fishes:—first, the ordinary bony fishes (Teleostians), such as the Cod, Salmon, and Herring; secondly, the Ganoid fishes, protected with bony plates on the skin, as the Bony-pike[37] and Sturgeon; thirdly, the Sharks and their allies, the Dog-fishes and Rays; fourthly, the peculiar and at present rare group of semi-reptilian fishes to which the name of _Dipnoi_ has been given, on account of their capacity for breathing both in air and in water. Of these four types the first is altogether modern, and includes the great majority of our present fishes. It does not make its appearance till the Cretaceous age, and then is at once represented by at least three of the modern families, those of the Salmon, Herring, and Perch. The history of the other three groups is precisely the opposite of this. They abound exceedingly at an early period, and dwindle to a much smaller number in the modern time. This is especially the case with the Ganoids and the Dipnoi. It is also remarkable that these groups of old-fashioned fishes[38] are in some respects the highest members of the class, approaching the nearest to the reptiles; but this accords with a well-known palæontological law, namely, that the higher members of low groups give way on the introduction of more elevated types, while the lower members may continue. Thus the decadence of these higher fish begins with the incoming of the reptiles, just as the decadence of the higher Mollusks and predaceous Crustaceans began with the incoming of the fishes. Further, the modern Ganoids and Dipnoi are mostly fresh-water animals, though the Sharks are largely pelagic. In the Palæozoic there seem to have been abundance of marine species of all these types; but though marine, they probably flourished most in bays and estuaries and on shallow banks; and the existence of these implies continental masses of land. This explains the curious coincidence that the introduction of fishes and of an abundant land flora synchronise, and that the ocean was still dominated by Invertebrates long after the fishes had become supreme in bays, estuaries, and rivers.