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Updated: June 29, 2025
In the vertebrate stem it was unquestionably a branch of the fishes in fact, of the Ganoids that made the first fortunate experiment during the Devonian period of adapting themselves to terrestrial life and breathing the atmosphere. This led to a modification of the heart and the nose.
They retained the evidence of their close relationship with the Devonian fishes in their cold blood, their gills and aquatic habit during their larval stage, their teeth with dentine infolded like those of the Devonian ganoids but still more intricately, and their biconcave vertebrae which never completely ossified.
On the Volga below Nijni Novgorod the sturgeon, and others of the same family, as also a very great variety of ganoids and Teleostei, appear in such quantities that they give occupation to nearly 100,000 people. The mouths of the Caspian rivers are especially celebrated for their wealth of fish. Siberia is emphatically the "Land of the North."
Lepidotus Mantelli, Agassiz. Wealden. a. Palate and teeth. b. Side view of teeth. c. The fishes of the Wealden are chiefly referable to the Ganoid and Placoid orders. These ganoids were allied to the Lepidosteus, or Gar-pike, of the American rivers. The whole body was covered with large rhomboidal scales, very thick, and having the exposed part coated with enamel.
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.
The ganoids survive in a few species out of thousands. The amphibia of the carboniferous and the next period and the reptiles of the mesozoic have disappeared; only a few feeble degenerate remnants persist. And this was necessarily so. Each advancing form crowded hardest on those which occupied the same place and sought the same food, that is, the members of the same species.
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.
These North American and African Ganoids are quite exceptional in the living creation; they are entirely confined to the northern hemisphere, unless some species of Polypterus range to the south of the line in Africa; and, out of about 9000 living species of fish known to M. Gunther, and of which more than 6000 are now preserved in the British Museum, they probably constitute no more than nine.
Again, vast as is the number of undoubtedly Ganoid fossil Fishes, and great as is their range in time, a large mass of evidence has recently been adduced to show that almost all those respecting which we possess sufficient information, are referable to the same sub-ordinal groups as the existing Lepidosteus, Polypterus, and Sturgeon; and that a singular relation obtains between the older and the younger Fishes; the former, the Devonian Ganoids, being almost all members of the same sub-order as Polypterus, while the Mesozoic Ganoids are almost all similarly allied to Lepidosteus.
The ganoids stand intermediate between the selaceans and teleosteans; the latter at the present day are largely preponderant in number; but formerly selaceans and ganoids alone existed; and in this case, according to the standard of highness chosen, so will it be said that fishes have advanced or retrograded in organisation.
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