242 



FISHES 



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 

 otlier Elasmobranchs the 

 pectoral fin of Cladoselache 

 (Fig. 145, A) is far more 

 primitive than in any other 

 Fish. Each fin is supported 

 by a distal series of slender, 

 more or less parallel, mi- 

 jointed, cartilaginous 

 radialia, 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."*^ Pleuracanthus, on the 

 other hand, had a biserial fin, the preaxial and postaxial radialia 

 supporting fan-like clusters of horny fibres at their distal ends 

 (Fig. 250). 



The broadly lobate pectoral fin of the existing Crossopterygii 



1 Bashford Dean, Anat. Anz. xi. 1896, p. 673. 



Fig. 145. — A, Pectoral fin, ami B, pelvic fin of 

 Clctdoselache. (From Bashford Dean.) 



