XVIII CHONDROSTEI 491 
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. Body 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 fulera.’ Rostrum 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 
Fic. 289.—Polyodon folium. a, Anus; jf, fulera; m, nostrils ; op, 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 foliwm (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. 
