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 
Fic. 69.—Heterccercal Tail, showing (A) external form and 
(B) internal structure. 
Fic. 70.—Vertebrated but symmetrical fin diy hycercal), 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); inthe latter the 
