ORDER SELACHOSTOMI — THE PADDLE-FISHES IS 



Order SELACHOSTOMI 



(the PADDLE-FISHES) 



Skeleton chiefly cartilaginous; the notochord persistent and the ver- 

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

 out spines, the ventrals abdominal; a mesoeoracoid 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, Polyodontidce. 



Family POLYODONTID.E 



(the paddle-fishes) 



Fishes with smooth" skin, and with the snout prolonged and ex- 

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

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

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

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

 mesoeoracoid 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 ehiasma; mouth broad, terminal, shark-like, the cleft dee] i, 

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

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

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

 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 o opha ;u arterial bulb with several pairs ol 

 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 Psephurus 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 Polyodontidce are represented by the head 



* The upper 1"1"' of the tail has .1 trace oi the primitive rh< 1 



