GANOIDEI. 



391 



cation of its mucous membrane, forming a spiral valve, such as 

 we shall afterwards see in the Sharks and Rnys. 



The order of the Ganoid fishes is divided by Owen into the 

 two divisions of the Lepidoganoidei and the Placoganoidei. The 

 best-known living fishes belonging to the Lepidoganoids are the 

 Bony Pike and the Polypterus. The Bony Pike {Lepidostens) 

 inhabits the rivers and lakes of North America, and attains a 

 length of several feet. The body is entirely clothed with an 

 armour of ganoid scales, arranged in obliquely transverse rows. 

 The vertebral column is exceedingly well ossified, and is rep- 

 tilian in its characters, the bodies of the vertebrae being " opis- 

 thoccelous." The jaws form a long narrow snout, armed with 

 a double series of teeth ; and the tail is heterocercal. 



Fig. I46,r-Oanoid Fishes. A, Polypterus; B, Oateale^U (extinct), a One of the pec- 

 toral fins, showing the fin-rays arranged round a central lobe ; b One of the ventral 

 fins ; c Anal lin ; d Dorsal fin ; tS Second dorsal lin. 



The Polypteri, of which several species are known, inhabit 

 the Nile, Senegal, and other African rivers, and are remarkable 

 for the peculiar structure, of the dorsal fin (fig. 146, A), which 

 is broken up into a number of separate portions, each composed 

 of a single spine in front, with a soft fin attached to it behind. 

 Two speci€(s oi Polypterus have recently been stated to^ossess 

 external branchiae when young, losing them when fully grown. 

 This observation, if confirmed, will bring the Ganoids into a 

 nearer relationship with the Mud-fishes (Lepidosiren). Another 

 group of Lepidoganoids is formed by the Trout-like Amia of 

 the fresh waters bf the United States. 



The section Placoganoidei includes the largest and best known 



