7 6 



PAIRED FINS 



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



A. i!^ 



""firs. 



Fig. 48. 



Dorsal lins, with tlie endoskeleton exposed, of: A, Notidanus (Heptanchus) cinereus, Gm. ; 

 B, Ginglymostoma cirratv/m, Gm. ; C,Zygaena malleus, Risso ; D, Bhynchdbatus djeddensis, Forsk. 

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

 longitudinal basal ; Its, basal ; il.s, distal segment of radial ; /, tin web supported by ceratotricnia ; 

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

 figures illustrate the formation of basals by the concrescence of radials. 



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

 front, also T lie endoskeleton of the two dorsal fins, 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 



