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 
fins become multi-basal. In the Chondrostei and the Holostei 
a metapterygium is always recognisable, supporting several 
radialia along its preaxial border, as in Aecipenser (Fig. 146, 
D) and Amia (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. 
Fic, 147,The left pectoral fin Of living Dipnoids Neoceratodus has a 
of Neoceratodus. a, b, First nearly typical biserial fin, but, as seems 
two segments of the axis ; LS, s ¢ 
preaxial horny fibres; +, +, 60 be the case in all fins of this type at 
pre- and post-axial radialia. present known, there is a marked ab- 
(After Wiedersheim. ) y 
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. Protopterus 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. 
