. The Cambridge natural history. Zoology. 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. 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, s
. The Cambridge natural history. Zoology. 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. 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.—Pobjodonfolium, 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 siz
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
