PELVIC FINS 245 



The Pelvic Fins. — In the simplicity of their endoslceletal 

 supports the pelvic fins of Cladoselache are the most primitive 

 type of paired fins at present known (Fig. 145, B). In general 

 structure they resemble the pectorals, but the radialia are fewer 

 in number, less modified by concentration, and exhibit little, 

 if any, trace of basal fusion. Add to such features as these 

 the apparent absence of any trace of pelvic rudiments, or of 

 basipterygia, and it will be obvious that the pelvic fins differ 

 but little from the median fins of the same Fish except that 

 they are paired. In Pleuracanthus the pelvic fins differ from 

 the corresponding pectorals iti being uniserial instead of biserial 

 (Fig. 250). All other 



Elasmobranchs, including 3 



the Holocephali, have uni- 

 serial fins, which consist 

 of a large metapterygium, c^^ 



supporting a preaxial ^^^^^^^-'^ty^^^A-V^/ / X' '^7-' 

 fringe of segmented 

 radialia. A proptery- 

 gium is sometimes present, 



, , 1 . „p +■!,„ Fig. 148. — Skeleton of a pelvic fin of Pohjodon 



notably m some of the ^^^.^^^_ ^^^^^^^^ ^.^^^ ^^.i;, y^^ ^^^^^j^, ^,„gi^ 



Skates and EayS, and, like ot the fin to the right ; to show the partial 



, , , . . , • fusion of the proximal portions of primitively 



tne metapterygium, it is distinct radialia to form a basipterygium. b, 



directly connected with Inner or mesial extremity of the basipterygium ; 



, . . ,, (Z.», dorsally directed, rudimentary iliac process ; 



the pelvic girdle. ,„^ foramenfor nerves. (After Rautenfeld.) 



The skeleton of the 

 pelvic fins of the Teleostomi is often extremely degenerate. 

 It is perhaps best developed in the Chondrostei,i ^here each 

 fin is supported by numerous segmented radialia, more or 

 fewer of which fuse towards the base of the fin, and those 

 form a large and slightly ossified basipterygium (Fig. 148). 

 In the living Crossopterygii, Holostei, and Teleostei, the pelvic 

 fins are similar in essential structure, but are very degenerate. 

 The basipterygium is usually well developed and is always bony 

 (Fig. 149), and in many Teleosts it acquires so extensive a 

 sutural connexion with its fellow that, physiologically, it supplies 

 the place of a true pelvic girdle. At its distal end there may 

 he a single row of small cartilaginous or bony nodules, repre- 

 senting vestigial radialia, as in the Crossopterygii, Holostei, and 



^ Thacker, Trans. Connecticut Acad. iv. 1877, p. 233. 



