ORDER SELACHOSTOMI THE PADDLE-FISHES 15 



Order SELACHOSTOMI 



(the paddle-fishes) 



Skeleton chieflv cartilaginous; the notochord persistent and the ver- 

 tebrae imperfectly formed, acentrous; anterior vertebrae single; fins with- 

 out spines, the ventrals abdominal; a mesocoracoid arch present; a feeble 

 suboperculum and a small rayed operculum; maxillary obsolete; air- 

 bladder cellular, with open duct. Fresh-water fishes of large size, in- 

 habiting rivers of North America and China. The order contains but 

 one family, PolyodontidcB. 



Family POLYODONTIDiE 



(the paddle-fishes) 



Fishes Avith smooth^'' skin, and with the snout prolonged and ex- 

 panded into a thin flat blade or paddle; notochord persistent; skeleton 

 chiefly cartilaginous, the vertebral column entirely so; the division into 

 vertebrae imperfect; ventral fins abdominal; dorsal and anal fins far 

 back; tail heterocercal, the caudal fin with fulcra; pectorals low; a 

 mesocoracoid arch present ; gills 4^ ; spiracles present ; spiracular pseudo- 

 branch vestigial or obsolete; no opercular gill; a single broad branchi- 

 ostegal; a small operculum present; suboperculum feeble and interoper- 

 culum obsolete; nostrils double, situated at base of blade; optic nerves 

 forming a solid chiasma ; mouth broad, terminal, shark-like, the cleft deep, 

 and overhung by the paddle-shaped snout; border of mouth formed by 

 premaxillaries, the maxillaries being obsolete; two pairs of minute bar- 

 bels situated on the under side of the rostrum in front of the mouth ; jaws 

 and palatines, in younger specimens, with numerous fine deciduous 

 teeth; intestine with a spiral valve; pyloric caeca present, in the form of a 

 broad, branching, leaf-like organ; air-bladder cellular, not bifid, con- 

 nected by a duct with the oesophagus; arterial bulb with several pairs of 

 valves. 



This family is represented by but two genera, each containing a 

 single species. These are Polyodon spathula, the paddle-fish of the 

 Mississippi Valley, and Psephiirus gladius, found in the valley of the 

 Yang-tse-Kiang in China. The latter species is said to reach a 

 length of 20 feet. Fossil Polyodontidcu are represented by the head 



* The upper lobe of the tail has a trace of the primitive rhombic scale-covering. 



