7 6 



PAIRED FINS 



arisen in the same way can scarcely be doubted. Indeed, it is 



B. 



.ar. 



abs. 



fir 



FIG. 48. 



Dorsal fins, with the ewloskeleton exposed, of: A, Xotidanus (Heptanchus) cinereits, Gin. ; 

 B, Ginglymostomu clrmtum, Gm. ; C,Zygaena malleus, Risso ; D, Rhynchobatus djeddensis, Forsk. 

 (After Mivart.) ate, anterior radials lying on vertebral column ; a.r, anterior radial ; 6, 

 longitudinal basal ; bs, basal ; tl.s, distal segment of radial ; /, fin web supported by ceratotrichia ; 

 m.s, median segment of radial ; p.r, posterior radial ; pr.s, proximal segment of radial. These 

 figures illustrate the formation of ba.sals by the concrescence of radials. 



FIG. 49. 



Left-side view of a portion of the, tail of Raja. The vertebral column has been exposed in 

 front, also the endoskeleton of the two dorsal tins, showing the concrescence of the radials at 

 their base to form a posterior axis. 



necessary to assume this even on Gegenbaur's theory, which derives 

 the fin-skeleton from originally separate gill-rays. 



The objection often raised that, in Elasmobranchs, the fin-radials 



