A C T1N0P TER 1 7 GII 



3°3 



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



rhipidostychous plan, with 



a postaxial axis (Fig. 279) ; ^_^ ff 



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, which 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- 



heim [492], Regan [345]) 



rather than from an out- 

 standing axis in a projecting 



lobe (see p. 108). Dorsal processes, analogous if not homologous 



with the iliac process, are 

 developed along the basal 

 region of the fin of Polyo- 

 don. In Scaphirhynchus 

 cataphradus 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 

 FlG - '-'" '■'• as in the lower Teleostomi 



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



view. (From Gegenbaur, Ve/rgl. Anat.) B, postaxial . ■, , -, ■, , 



edge of fin-skeleton : cZ, clavicle ; cl', cleithrum ; F, IS neterocercal, llOmOCercal, 



base of left fin ; m, muscle ; JJ, enlarged preaxial dermal „ v rrar\VnTiv\nn¥v>al „ nro - 



ray ; r, endoskeletal radials. ul gepiiyiuceicdl, ne\ei 



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 



Flo. 278. 



Ventral view of the pelvic girdle and fins of 

 Acipenser sturio, L. The skeleton of the right tin is 

 completely exposed. dr, lepidotrichia supporting 

 web of fin ; p, pelvic cartilage ; pr.r, preaxial radials. 

 (From Quart. Jovrn. Micr. Sei.) 



