242 FISHES CHAP. 
plate, produced in front into a long tapering epipubic process, 
and on each side of this into a forwardly inclined prepubic pro- 
cess. The hinder part of the plate bears two short processes for 
the basal cartilages of the pelvic fins. There is no trace, how- 
ever, of iliac processes. 
The Pectoral Fins.—The skeleton of the pectoral fins exhibits 
remarkable structural variations in different Elasmobranchs. In 
the existing members of the group two large basal cartilages, the 
propterygium and the mesopterygium, are formed by the concen- 
tration and fusion of the proximal portions of certain of the 
preaxial radialia, and they, with the metapterygium, articulate 
with the pectoral girdle; 
hence the fin is_ tribasal 
as well as uniserial (Figs. 
141 and 146, A, B). In 
striking contrast to all 
other Elasmobranchs — the 
pectoral fin of Cladoselache 
(Fig. 145, A) is far more 
primitive than in any other 
Fish. ach fin is supported 
by a distal series of slender, 
more or less parallel, un- 
jointed, cartilaginous 
radiaha, and basally by a 
similar series of shorter, 
stouter, and less numerous 
cartilages, which apparently 
were imbedded in the body- 
wall, the entire fin skeleton 
presenting a striking  re- 
semblance to an _ isolated 
median fin in which the supporting radialia have concentrated 
by growth pressure, and their proximal portions have been 
reduced in number by partial fusion.’ Plewracanthus, on the 
other hand, had a biserial fin, the preaxial and postaxial radiaha 
supporting fan-like clusters of horny fibres at their distal ends 
(Fig. 250). 
The broadly lobate pectoral fin of the existing Crossopterygil 
Fic. 145.—A, Pectoral fin, and B, pelvic fin of 
Cladoselache. (From Bashford Dean.) 
1 Bashford Dean, Anat. Anz. xi. 1896, p. 673. 
