170 Darwin, and after Darwin. 



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

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

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

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



FlG. 69. Heterocercal Tail, showing (A) external form and 

 (B) internal structure. 





FlG. 70. Vertebrated but symmetrical fin (diphycercal), showing 

 (A) external form and (B) internal structure. 



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

 fundamental than in form. 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 



