. Fishes. Fishes. 250 The Ganoids The Palaeoniscidse.—The numerous genera of this order are referred to three families, the Palcuoniscidcs, PlatysomidcB, and Dictyopygidm; a fourth family, Dorypteridw, of uncertain re- lations, being also tentatively recognized. The family of PalcBoniscidcB is the most primitive, ranging from the Devonian to the Lias, and some of them seem to have entered fresh waters in the time of the coal-measures. These fishes have the body elongate and provided with one short dorsal fin. The tail is heterocercal and the body covered with rhombic plates. Fulcra or rudiment
. Fishes. Fishes. 250 The Ganoids The Palaeoniscidse.—The numerous genera of this order are referred to three families, the Palcuoniscidcs, PlatysomidcB, and Dictyopygidm; a fourth family, Dorypteridw, of uncertain re- lations, being also tentatively recognized. The family of PalcBoniscidcB is the most primitive, ranging from the Devonian to the Lias, and some of them seem to have entered fresh waters in the time of the coal-measures. These fishes have the body elongate and provided with one short dorsal fin. The tail is heterocercal and the body covered with rhombic plates. Fulcra or rudimentary spine-like scales are developed on the upper edge of the caudal fin in most recent Ganoids, and often the back has a median row of undeveloped scales. A multi- tude of species and genera are recorded A typical form is the genus PalcBonisctmt* with many species represented in the rocks of various parts of the world. The longest known species is PalcBoniscmn frieslebenense from the Permian of Germany and England. Palceoniscum magnum, sixteen inches long, occurs. Fig. 185.—Palceoniscum frieslebenense Blainville. Family Palaoniscidce (After Zittel.) in the Permian of Germany. From Canobiiis, the most primi- tive genus, to Coccolepis, the most modern, is a continuous series, the suspensorium of the lower jaw becoming more obHque, the basal bones of the dorsal fewer, the dorsal extending farther forward, and the scales more completely imbricate. Other prominent genera are Amblypterus, Eurylepis, Cheirolepis, Rhadinichthys, Pygopterus, Elonichthys, Mrolepis, Gyrolepis, Myriolepis, Oxygnathus, Centrolepis, and Holurus. The Platysomidse. — The Platysomidcs are different in foim, the body being deep and compressed, often diamond-shaped] * This word is usually written Palcsoniscus, but Blainville, its author (i8i8), chose the neuter Please note that these images are extracted from scanned page images that may have been digitally enhanced for readability - coloration and a
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