250 The Ganoids 
The Palzoniscide.—The numerous genera of this order are 
referred to three families, the Palgoniscide, Platysomide, and 
Dictyopygide; a fourth family, Dorypteride, of uncertain re- 
lations, being also tentatively recognized. The family of 
Palgoniscide 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 Palgoniscum,* with many species represented in the 
rocks of various parts of the world. The longest known species 
is Paleoniscum frieslebenense from the Permian of Germany 
and England. Paleoniscum magnum, sixteen inches long, occurs 
Fig. 185.—Paleoniscum frieslebenense Blainville. Family Paleoniscide. 
(After Zittel.) 
in the Permian of Germany. From Canobius, the most primi- 
tive genus, to Coccolepis, the most modern, is a continuous series, 
the suspensorium of the lower jaw becoming more oblique, 
the basal bones of the dorsal fewer, the dorsal extending farther 
forward, and the scales more completely imbricate. Other 
prominent genera are Amblypterus, Eurylepis, Chetrolepis, 
Rhadinichthys, Pygopterus, Elonichthys, Atrolepis, Gyrolepis, 
Myrtolepis, Oxygnathus, Centrolepis, and Holurus. 
The Platysomide.— The Platysomide are different in form, 
the body being deep and compressed, often diamond-shaped, 
* This word is usually written Palgoniscus, but Blainville, its author (1818), 
chose the neuter form. 
