DEVELOPMENT OF TAIL. 



149 



aud a further development of this iiuMjuality results in the 

 partially.forked tail of the sharks, where the end of the 

 backbone is bent upwards into the upper and longer 

 half of the tail, the lower lobe of which is formed solely 

 of rays. Sharks and lung-iishes have, indeed, never 

 advanced beyond one or other of these two modifications of 

 the fringed-tailed type. On the other hand, the com- 

 pound group, including the ganoids and the bony fishes, 

 was by no means satisfied with the primitive arrangement 

 of matters. Starting from a fish of the fringe-tailed type 

 like the one 

 represented in 

 Fig. 48, we 

 may trace a 

 gradual short- 

 ening of the 

 central part of 

 the tail - fin, 

 accompanied 

 by an increas- 

 ing develo])- 

 ment of tile 

 rays on its 

 lower side, im- ' 

 til we finally 

 reach the com- 

 pletely-forked 

 tail of the 

 perch (Fig. 



49), in which, as we have seen, the backbone stops 

 short of the fin-rays, and ends in an expanded ex- 

 tremity from which these rays are given off in a fan-like 

 manner. The bony fishes have, therefore, not only 

 succeeded in developing the fringed-fiiis of their ancestral 

 ganoids into those of a fan-like type, but have likewise 

 effected a precisely similar modification in the structure 

 of their tails. I'hat the fan-like tail of the perch is an 

 improvement as a steering organ upion the fringed tail of 

 the early ganoids there can be no doubt ; and it is such an 

 organ which alone could regulate the movements of the 



Fio. .52.— Right Upper Tootli of an Extinct 

 Liing-Pisli (Cercdodus). C. point of contiu't 

 with opposite tooth. (After Teller.) 



