CHONDROSTEI 49 1 



as the outcome of a long-continued career of degeneration from 

 some remote Palaeoniscid ancestor. 



Fam. 6. Polyodontidae. — The Polyodontidae are more 

 generalised, and in some features decidedly more Selachioid 

 than the Acipenseridae. Bo'dy fusiform and apparently scaleless, 

 but the primitive squamation is still represented by isolated 

 vestigial scales imbedded in the otherwise soft skin, and by a 

 continuous series of rhombic scales on the upper caudal lobe, 

 which also has a dorsal fringe of large fulcra.^ Eostrum excep- 

 tionally long, spatulate or somewhat conical, with a rigid axis 

 and thinner and more flexible margins. Barbels absent. Mouth 

 wide, not spout-like. Pectoral fins devoid of spines. Two pairs 

 of membrane-closed vacuities separate the paired dermal bones of 

 the cranial roof (possibly parietals and frontals) from the more 



Fig. 289.— Pobjodon folium, a, Anus ; /, fulcra ; n, nostrils ; o}}, operculum ; 

 sc, rhombic scales on the upper caudal lobe ; sp, left spiracle. 



laterally-placed post-temporals and squamosals, and there are no 

 median plates posterior to the orbits, nor any representatives of 

 supra-temporals. A feeble suboperculum is retained in addition 

 to a small rayed operculum. Hyoidean hemibranch completely sup- 

 pressed. Two genera only are known, each with a single species. 

 The Paddle-Fish or Spoon-Bill, Polyodon folium (Fig. 289) 

 inhabits the rivers of the Southern States of North America, the 

 Mississippi, Ohio, and Missouri, and their numerous tributary 

 rivers and streams. A Fish of sluggish habits, Polyodon feeds 

 chiefly on mud and the minute organisms it contains, the excep- 

 tionally long gill-rakers probably forming an efficient filter to 

 prevent the food particles escaping through the gill-clefts with 

 the expiratory water current. The singular rostrum is appar- 

 ently used for stirring up the mud when feeding, but in view of 

 the muddy waters the Fish frequents, and the very small size of 



1 Jordan and Evermann, "Fishes of North and Middle America," Bull. U.S. 

 Nat. Mus. No. 47, Pt. i. 1896, p. 101. 



