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Page:Popular Science Monthly Volume 21.djvu/397

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colony of Queensland, accounts were given of the existence in the rivers of the country of a large fish which the colonists called a "salmon," from the fact of its having salmon-colored flesh, and of sometimes, it was said, rising to a fly. Mr. Krefft, Curator of the Australian Museum, at Sydney, having examined a specimen of this fish in 1869 or 1870, pronounced it to be allied to the mud-fish, but discovered also that it possessed teeth so closely resembling those of certain fossil fishes attributed by Agassiz to the sharks, that no doubt could be entertained of the generic identity of the two forms. Accordingly, the newly found animal was described as a species of Ceratodus—that being the name which Agassiz had conferred on the creatures whose fossil teeth he had long before made known. Now, all this was in itself sufficiently remarkable, for it proved that Ceratodus, as a genus, had persisted from the Mesozoic era; but its important bearing was not fully perceived till after some more examples had been obtained and sent to Dr. Günther. He described the recent Ceratodus in great detail in the "Philosophical Transactions" for 1871, and was able, furthermore, by its means, to show how the palæozoic as well as the recent ganoids, the sharks and skates—both ancient and modern—the sturgeons, the mud-fishes, with some other forms that had hitherto been irreconcilable, could all be brought together through some essential characters common to the whole of them, and harmoniously placed in a single class, to which he assigned the name of "Palæichthyes"—fishes of ancient type. Professor Huxley had previously pointed out the affinity of the mud-fishes to certain ganoids, but the credit of discovering this comprehensive classification is due to Dr. Günther.

Dr. Günther does not undertake to describe definitely the geographical distribution of fishes in the sense in which the term geographical distribution is used by naturalists of other branches, he holding that "the endeavor to establish, by means of our present fragmentary geological knowledge, the divisions of the fauna of the globe leads us into a maze of conflicting evidence." It is obvious that fishes are not amenable to the laws of geographical distribution which govern land animals. In treating of their relations in this respect it is, moreover, necessary to separate them into categories, of which Dr. Günther makes four: 1. Fresh-water fishes; 2. Brackish-water fishes; 3. Marine fishes, which are furthermore subdivided into shore-fishes and oceanic or pelagic fishes; and, 4. Fishes of the deep sea. Even in the case of fresh-water fishes, which of course live under conditions more similar to those of land-animals than do those of the other categories, he disallows the six great zoögeographical regions which most geologists have accepted, and would arrange them in three zones—Northern, Equatorial, and Southern. These zones, are, however, broken up into regions, which roughly correspond with the six generally received, except that the Australian region of most zoögeographers is split up into