GANOIDEI. 



367 



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

 we shall afterwards see in the Sharks and Rays. 



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 (Lcpidosteus) 

 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. 138. Ganoid Fishes. A, Polypterus', B, Osteolepis (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 fin ; d Dorsal fin ; d Second dorsal fin. 



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. 138, 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 species of Polypterus have recently been stated to possess 

 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). 



The section Placoganoidei includes the largest and best known 

 of all the living Ganoid fishes namely, the Sturgeons and it 

 also contains some highly singular fossil forms. The sub- order 

 is defined by the fact that the skeleton is always imperfectly 



