GANOIDEI. 



4*5 



VII. The intestine is often furnished with a spiral redupli- 

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

 as we shall afterwards see in the Sharks and Rays. 



The classification of the Ganoid Fishes has hitherto proved a 

 matter of extreme difficulty ; and probably no arrangement 

 that has been as yet proposed can be regarded as being, in all 

 its details, more than provisional. A convenient primary 

 division is that into Lepidoganoids, in which the body is 

 furnished with scales of moderate size, and the endoskeleton 

 is generally more or less perfectly ossified, and Placoganoids, 

 in which the skeleton is imperfectly ossified, and the head and 

 more or less of the body are protected by large ganoid plates, 

 which in many cases are united together by sutures. Accept- 



1 . Fig. 175. 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. 



ing this division, the order Ganoidei may be divided into the 

 following sub-orders : 



SECTION i. LEPIDOGANOIDEI. 

 Sub-order A. Amiada. (Recent.) 



- B. Lepidosteidce . (Recent.) 



C. Lepidopleuridce. (Extinct.) 

 D. Crossopterygidce. (Recent.) 



E. Acanthodidce, (Extinct.) 

 SECTION 2. PLACOGANOIDEI. 



Sub-order F. Ostracostei. (Extinct.) 

 G. Sturionidcs. (Recent.) 



The best-known living fishes belonging to the Lepidoganoids 

 are the Bony Pike and the Polypterus. The Bony Pike 



