68 THE STORY OF FISH LIFE. 



But other modifications of the fin took place 

 at about this time, and the rays of the primitive 

 supporting rods grew stronger on one side of the 

 axis than the other, whilst the axis itself became 

 slowly transformed, ultimately resulting in a 

 series of flattened plates supporting jointed 

 cartilaginous rods, fringed by the hair-like rays 

 already described. Fins of this kind are present 

 in our modern sharks and dog-fishes (fig. 7, E.). 

 This form of fin in turn became modified into 

 that which we find in the typical bony fishes such 

 as the perch, pike or cod-fish ; and in the ancient 

 but still surviving " Bichir " or Polypterus of 

 the Nile. 



In examining the fins of the perch, either the 

 median or paired fins, we should miss the 

 hair-like rays which fringe the border of the fin, 

 and we should find in the dorsal fin, for instance, 

 as we have already noticed (p. 60), that the fin 

 supports were solid and bony and rested upon 

 smaller spike-like bony supports Avhich in turn 

 were connected with, and corresponded in 

 number with, the spines of separate vertebrae of 

 the vertebral column. It is generally believed 

 that these external bony fin-supports have been 

 formed by the fusion of clusters of these original 

 hair-like rays, the hair-like stage preceding the 

 osseous rod-stage. 



Thus, by insensible gradations, we may trace 

 the origin and evolution of the fins of fishes. 

 Let us recapitulate these stages. First then to 

 arise are the vertical fins. These being profit- 

 able to the fish lead to a further extension of 

 the fin system by the addition of lateral folds. 



