ORDER SELACHOSTOMI — THE PADDLE-FISHES 15 



Ordeb SELACHOSTOMI 



THE PADDLE-FISHES 



Skeleton chiefly cartilaginous; the notochord persistent and the ver- 

 tebrae imperfectly formed, acentrous; anterior vertebrae single; fins without 

 spines, the ventrals abdominal; a mesocoracoid arch present; a feeble sub- 

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

 cellular, with open duct. Fresh-water fishes of large size, inhabiting rivers 

 of North America and China. The order contains but one family, Pol- 

 yodontidce. 



Family POLYODONTID/E 



THE PADDLE-FISHES 



Fishes with smooth* skin, and with the snout prolonged and expanded 

 into a thin flat blade or paddle; notochord persistent; skeleton chiefly car- 

 tilaginous, the vertebral column entirely so ; the division into vertebrae im- 

 perfect; ventral fins abdominal; dorsal and anal fins far back; tail hetero- 

 cercal, the caudal fin with fulcra; pectorals low; a mesocoracoid arch present; 

 gills 43^; spiracles present; spiracular pseudobranch vestigial or obsolete; 

 no opercular gill; a single broad branchiostegal ; a small operculum present; 

 suboperculum feeble and interoperculum obsolete; nostrils double, situated 

 at base of blade; optic nerves forming a solid chiasma; mouth broad, ter- 

 minal, 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 barbels situated on the under side of the rostrum in 

 front of the mouth; jaws and palatines, in younger specimens, with nu- 

 merous 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, connected by a duct with the oesophagus; arterial bulb with several 

 pairs of valves. 



This family is represented by but two genera, each con- 

 taining 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 Polyodon- 

 tidce are represented by the head and caudal region of a form 



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



