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In the genera Dipterus and Diplopterus, as Hugh Miller pointed out, and in several other of the fringe-finned genera, as in Gyroptychius and Glyptolepis, the two dorsals are placed far backward, or directly over the ventral and anal fins. The Asterolepis was a ganoid fish of gigantic dimensions.
If we except the Placoids already alluded to, and a few other families of doubtful affinities, all the Old Red Sandstone fishes are Ganoids, an order so named by Agassiz from the shining outer surface of their scales; but Professor Huxley has also called our attention to the fact that, while a few of the primary and the great majority of the secondary Ganoids resemble the living bony pike, Lepidosteus, or the Amia, genera now found in North American rivers, and one of them, Lepidosteus, extending as far south as Guatemala, the Crossopterygii, or fringe-finned Ichthyolites, of the Old Red are closely related to the African Polypterus, which is represented by five or six species now inhabiting the Nile and the rivers of Senegal.
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