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 

 ■^ 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?.?, , , \,^ ^ . „ „ o ^r. ■ ., 



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 in Lepidosiren even the preaxial have 

 atrophied, leaving only the long jointed axis to represent the 

 originally biserial fin. 



