PELVIC FINS 



245 



The Pelvic Fins. In the simplicity of their endoskeletal 

 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 in being uniserial instead of biserial 

 (Fig. 250). All other 

 Elasmobranchs, including 

 the Holocephali, have uni- 

 serial fins, which consist 

 of a large metapterygium, 

 supporting a preaxial 

 fringe of segmented 

 radialia. A proptery- 

 gium is sometimes present, 

 notably in some of the 

 Skates and Bays, and, like 

 the metapterygium, it is 

 directly connected with 

 the pelvic girdle. 



The skeleton of the 



pelvic fins of the Teleostomi is often extremely degenerate. 

 It is perhaps best developed in the Chondrostei, 1 where 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 

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

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

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



FIG. 148. Skeleton of a pelvic fiii of Polyodon 

 folium, ventral view, with the anterior margin 

 of the fin to the right ; to show the partial 

 fusion of the proximal portions of primitively 

 distinct radialia to form a basipterygium. b, 

 Inner or mesial extremity of the basipterygium ; 

 d.p, dorsally directed, rudimentary iliac process ; 

 n, foramen for nerves. (After Rautenfeld.) 



