Class PISCES. 



Skeleton more or less ossified; skull well developed, separate from 

 the segmented vertebral column; mouth with true jaws; limbs, 

 shoulder, and pelvic bones usually present; usually less than 6 gill 

 openings on each side; gills attached to imperfect skeletal arches; 

 nostrils one or more pairs ; heart with arterial bulb ; alimentary canal 

 variable in form, and with pancreas and spleen; tail heterocercal, 

 homocercal, or isocercal. 



Order Selachosfomi. 



The Paddle-fishes. 



Skeleton chiefly cartilaginous, the vertebrae imperfectly formed, 

 acentrous; anterior vertebras single; ventral fins abdominal; max- 

 illary obsolete; air bladder cellular, with open duct to oesophagus; 

 snout long and paddle-shaped; tail heterocercal. 



Family Polyodontidse. 



The Paddle-fishes. 



Body elongate, not covered with scales or bony plates; ventral fins 

 abdominal ; fins without spines ; opercle and subopercle small ; 

 mouth large, its border formed by premaxillaries ; maxillaries obsolete ; 

 snout long, paddle-shaped; 2 pairs of minute barbels on snout, just 

 in front of the mouth; jaws and palatines in young with numerous 

 fine deciduous teeth; intestine with spiral valve; pyloric coeca broad, 

 leaf-like ; arterial bulb with several pairs of valves. 



Genus Polyodoii Lacepede. 



Gill-rakers numerous, very long and slender; caudal fulcra of 

 moderate size, 13 to 20 in number; opercle produced into a long 

 triangular flap. 



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