14 The Ganoids 



The Palaeoniscidse. The numerous genera of this order are 

 referred to three families, the Palaoniscida, Platysomida, and 

 Dictyopygidcc ; a fourth family, Dorypterida?, of uncertain re- 

 lations, being also tentatively recognized. The family of 

 PalceoniscidcB 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 hetercoercal 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 Palaoniscum* with many species represented in the 

 rocks of various parts of the world. The longest known species 

 is PalcEoniscum frieslebenense from the Permian of Germany 

 and England. Pal&oniscum magnum, sixteen inches long, occurs 



FIG. 2. Palceoniscum frieslebenense Blainville. Family Palceoniscidce. 



(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, Cheirolepis, 

 Rhadinichthys, Pygopterus, Elonichthys, AZrolepis, Gyrolepis, 

 Myriolepis, Oxygnathus, Centrolepis, and Holurus. 



The Platysomidae. The Platysomida are different in form, 

 the body being deep and compressed, often diamond-shaped, 



* This word is usually written Pal&oniscus, but Blainville, its author (1818), 

 chose the neuter form. 



