1 70 Darwin, and after Darwin. 



It has long been noticed that there are among fishes two styles 

 of tail-fins. These are the even-lobcd, or homocercal (Fig. 68), 

 and the uneven-Iobed, or heterocercal (Fig. 69). The one is 

 characteristic of ordinary fishes (teleosts), the other of sharks 



Fig. 69. — Heterocercal Tail, showint; (A) external form and 

 (B) internal structure. 



Fig. 70. — Vertebrated but symmetrical fin ^diphycercal), showing 

 (A) external form and ;,B) inteinal btructure. 



and some other orders. In structure the difference is even more 

 fundamental than in forin. In the former style the backbone 

 stops abruptly in a series of short, enlarged joints, and thence 

 sends off rays to form the tail-fin (Fig. 68) ; in the latter the 



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