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The "true Fishes" are divided by him into Ctenoids and Cycloids, according as the posterior margin of their scales is denticulated or smooth, a circumstance the importance of which to the animal must be infinitely small, in comparison to the peculiarities of the dentition, formation of the fins, number of vertebrae, etc.
"More than fifteen hundred species of fossil fishes, which I have learned to know, tell me that species do not pass insensibly one into another, but that they appear and disappear unexpectedly, without direct relations with their precursors; for I think no one will seriously pretend that the numerous types of Cycloids and Ctenoids, almost all of which are contemporaneous with one an other, have descended from the Placoids and Ganoids.
I did both eagerly, and acquired a considerable knowledge of the literature of ichthyology, becoming especially interested in the system of classification, then most imperfect. I tried to follow Agassiz's scheme of division into the order of ctenoids and ganoids, with the result that I found one of my species of side-swimmers had cycloid scales on one side and ctenoid on the other.
It will be necessary to enter more at large than in my preceding volumes into the resources of Canada, and, for this end, Geology and other scientific subjects must be introduced; but, as I dislike exceedingly that heavy and gaudy veil of learning, that embroidered science, with which modern taste conceals those secrets of Nature which have been so partially unfolded, I shall not have frequent recourse to absurd Greek derivations, which are very commonly borrowed for the occasion from technical dictionaries, or lent by a classical friend; but, whenever they must occur, the dictionary shall explain them, for I really think it beneath the dignity of the lights of modern Geology to talk as they do about the Placoids and the Ganoids, as the first created fishlike beings, and of the Ctenoids and the Cycloids as the more recent finners.
Underlying all its divisions and subdivisions, is the average expression of the type in the past and present; the Placoids and Ganoids, with their combination of reptilian and fishlike features, characterizing the earlier geological epochs, while in the later the simple bony fishes, the Cycloids and Ctenoids, take the ascendancy.
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