. The Cambridge natural history. Zoology. 244 FISHES CHAP. gradually reduced in number and importance, their place as fin - supports being usurped by the dermal fin - rays. In addition, more than three, usually several, basal elements articulate directly with the pectoral girdle, and hence the iins become multi-basal. In the Chondrostei and the Holostei a metapterygium is always recognisable, supporting several radialia along its preaxial border, as in Acipenser (Pig. 146, D) and Ainia (Fig. 146, E), or only a single one, as in Lepidosteus (Fig. 146, F). The anterior part of the fin is support
. The Cambridge natural history. Zoology. 244 FISHES CHAP. gradually reduced in number and importance, their place as fin - supports being usurped by the dermal fin - rays. In addition, more than three, usually several, basal elements articulate directly with the pectoral girdle, and hence the iins become multi-basal. In the Chondrostei and the Holostei a metapterygium is always recognisable, supporting several radialia along its preaxial border, as in Acipenser (Pig. 146, D) and Ainia (Fig. 146, E), or only a single one, as in Lepidosteus (Fig. 146, F). The anterior part of the fin is supported by a variable number of cartilaginous or bony radialia, which, with the metapterygium, articulate with the limb-girdle. In Teleosts the pro- cess of reduction reaches its maximum. Usually there is but a single row of short, hour-glass-shaped ossicles, of which the postaxial one may represent a ves- tigial metapterygium, and sometimes â ^ there is also a distal row of small cartilages or ossicles, partially hidden in the cleft bases of the dermal fin- rays (Fig. 146, H). In all these Fishes the fin is a much reduced uniserial fin, in which more or fewer of the preaxial radialia have acquired a direct secondary connexion with the pectoral girdle. Pig. I47~^he left pectoral fin Of living Dipnoids Neoceratoclus has a of yeoceratodns. a, b, First nearly typical biscrial fin, but, as seems two segments of the axis ;i?.?, , , \,^ ^ . â â o ^r. â ., preaxial horny fibres ; t, f, tO ^6 the Case in ail fans 01 thlS type at pre- and post-axki radialia. present known, there is a marked ab- (After Wiedersheini.) '- sence of symmetry in the number and disposition of the radialia on the two sides of the axis. There is also much individual variation. No two fins are precisely alike, and the radialia may sometimes divide. In the very acutely lobate fins of the remaining Dipnoids it is evident that great reduction has taken place. Frotopterus has lost all trace of postaxial radialia, and
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Keywords: ., bookcentury1800, bookdecade1890, booksubjectzoology, bookyear1895
