ACTINOPTERYGII 



33 



The pectoral fin has a fairly normal skeleton, conforming to the 

 rhipidostychous plan, with 

 a postaxial axis (Fig. 279); 

 but the pelvic fin has a 

 remarkable structure, differ- 

 ing from that of all other 

 Osteichthyes. No definite 

 line of demarcation exists 

 between the pelvic girdle 

 and the skeleton of the fin 

 itself (Figs. 276-8). The 

 radials, forming a single 

 series, are articulated to 

 basals, Avhich are more or 

 less independent posteriorly, 

 but become fused to the 

 girdle in front. A skeleton 

 is thus formed somewhat 

 resembling that of Clado- 

 selache, and suggesting a 

 derivation from a primitive 

 orthostichous type (Wieders- Ai .. Jteiuer Sturi0t L . The s - ke ieto,rof the right im is 



heim [4921, Regan [3451) completely exposed. </r, lepidotrichia supporting 

 ' , \veb of fin ; p, pelvic cartilage ; }//. r, preaxial radials. 



rather than irom an out- (From Quart. Jovm. Micr. ad.) 



standing axis in a projecting 



Dorsal processes, analogous if not homologous 

 with the iliac process, are 

 developed along the basal 

 region of the fin of Polyo- 

 don. In Scaphirhynclws 

 cataphmctus the internal 

 end of the pelvic cartilage 

 may be segmented off as a 

 separate element. So far 

 as is known, the radials 

 of the dorsal and anal fins 

 undergo little concentra- 

 tion, and do not tend to 

 fuse together proximally 



Ventral view of the pelvic girdle and fins of 



lobe (see p. 108). 



a 



FIG. 27?. 



as in the lower Teleostomi 



Pectoral girdle and fins of Acipenscr sturio. Ventral and the Dipnoi. The tail 

 lew. (From Gegenbaur, Vergl Anal) IS, iwstaxial . v. f ..__ i hnmn^pr^l 



edge of fin-skeleton ; d, clavicle ; d', cleithrum ; F, 



base of left fin ; m, muscle ; R, enlarged preaxial dermal Qr geph VTOCercal never 



ray ; r, endoskeletal radials. o 1 J 



truly diphycercal (Belono- 



rhynchidae ?). When the median fin is subdivided, as is the 

 case in all except some Teleostei, a single anal and a single 



