FISHES 29 



bladder provided with an open duct connecting it with the pharynx or 

 the oesophagus; with an arterial bulb with several pairs of valves; and 

 with an intestinal spiral valve: 3 orders, with about 13 American species. 



Key to the Orders of Ganoidea 



ai Ganoids represented in Africa by the genus Po/y/>/en(^ but not 



in America and not inchided in this book j. Crossopterygii. 



a2 Ganoids without scales, with the body either naked or with 



bony plates 2. Chondrostei. 



as Ganoids with the body covered either with ganoid or cycloid 



scales 3 . Holostei. 



Order 2. Chondrostei. — Cartilaginous ganoids. Fish of large 

 size and with the skeleton mostly and the vertebral column entirely 

 cartilaginous; notochord persistent; opercles usually large; branch- 

 iostegal rays weak or wanting; mouth ventral beneath a long snout; 

 body naked or with bony plates; tail heterocercal : 2 families, with 8 

 American species. 



Key to the Families of Chondrostei 



ai Body naked i. Polyodontidce. 



ao Body more or less covered with bony plates; sturgeons 2. Acipenseridce. 



Family i. Polyodontidae. — The paddle-fishes. Body without 

 scales and smooth; snout spatulate and very long; teeth minute, present 

 only in the young; opercles rudimentary, prolonged behind on each 

 side to form a long flap; spiracle present; i branchiostegal ray; air 

 bladder cellular : 2 species and genera, i in China {Psephurus gladius) . 



Polyodon Lacepede. Gill-rakers very fine and numerous: i species. 



P. spathula (Walbaum). Spoonbill; paddle-fish. Extreme length 

 1.800 mm.; extreme weight 150 lbs.; color olivaceous; head, with 

 opercle, more than half the length of the body; spatula one-third the 

 total length: Mississippi Valley; French Broad, North Carolina; often 

 common in the larger streams and lakes, where it feeds on small organ- 

 isms in the mud; used for food and the roe made into caviar. 



Family 2. Acipenseridae.— Sturgeons. Large fish with 5 longi- 

 tudinal rows of keeled bony plates on the body, a middorsal series and a 

 lateral and an abdominal series on each side, between which are small 

 plates or granules; mouth ventral and toothless, in front of which pro- 

 jects a long snout; 4 barbels in a transverse row in front of the mouth; 

 gill-slits 4, with an accessory opercular gill; no branchiostegal rays; 

 ventral fins posterior; median fins with spine-like projections called 

 fulcra: about 20 species; 7 species and 3 genera in America. 



