Glass 2. Pisces. Order 3. Ganoidei. 385 



provided with bony plates or scales (ganoid scales) ; dermal denticles 

 may also, be present, but in small numbers. Bony rays occur in the 

 folding fins. 



This group was formerly very well represented; few of its members 

 are living, however, at the present day. 



Sub-Order 1. ChondrOStei {Cartilaginous Ganoids). 



The skeleton is, for the most part, cartilaginous j only membrane 

 bones are present. The mouth ventral. The tail heterocercal. 



1. Sturgeons (Accipenser) have five rows of large bony plates arranged 

 along tlie body (one row being median) dorsally, and manyS small plates 

 irregularly placed ; dorsally upon tte bead are large bony plates, which cover 

 the chondrocranium : the mouth is small, edentulous (the young, however, have 

 teeth, and sometimes small teeth occur on the gill-bars of the adult) ; on the 

 ventral side of the often elongate snout there are tactile tentacles : a spiracle 

 is present. A. sturio, which attains a length of several metres, inhabits North 

 European seas, wandering up into the rivers to spawn ; there are several other 

 species in the Caspian and Black Seas, and in the large rivers of Russia 

 (Sterlet, A. ruthenus, A. huso, etc.). 



2. The Spoon-billed Sturgeons (Spatularia) differ from 

 Accipenser in that the snout is prolonged into a large horizontal blade, and the 

 skin is almost without hard parts ; in the mouth weak teeth are developed. In 

 North American and Chinese rivers. 



Sub-Order 2. Holostei {Bo)/_// Ganoids.) 



The skeleton is for the most part ossified. The mouth anterior. 

 Large, rhomboidal, enamelled* scales, which may be partly dove- 

 tailed together, or, more rarely, scales like those of the Teleosteans. 

 Usually (Lepidosteus, Amia) the respiratory organ is a true lung. 

 All existing forms are freshwater. 



1. Polypterus. Long dorsal fin, with strong fin-rays, usually fan-shaped at 

 the tip, and not connected together ; no anal fin ; caudal fin rounded, feebly 

 heterocercal (the bent-up portion of the spinal column is very small). Large 

 rhomboidal scales. A spiracle. In Africa [e.g., in the Nile). 



2. The Bony Pike (Lepidosteus). Snout much elongated ; shoi't dorsal 

 and anal fins ; well-marked heterocercaUty, the caudal fin being almost entirely 



Fig. 315. Lepidosteus. 



* The scales are covered externally by a smooth layer, commonly called " enamel " ; 

 but it is not true enamel, like that of the teeth, it is only an external, polished, dense 

 layer of bone. 



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