56 



A HISTORY OF FISHES 



case of the median fins, the appearance of the continuous folds 

 must have been followed by the development of a series of rods 

 of cartilage designed to strengthen these structures. Shortly 

 afterwards, the middle portions of these folds degenerated and 

 disappeared, and the parts at either end became enlarged and 



A, B. 



Fig. 23. EVOLUTION OF FINS. 



Hypothetical primitive Shark-like fishes ; c. Blue Shark {CarcJiarimis 

 lamia) ; D. Cladoselache fyleri ; e. Climatius macnicoli. 



gradually took on the definitive form of the pectoral and pelvic 

 fins. The cause of these gaps is not known, but may have been in 

 some way connected with the undulating movements of the fish. 

 As pointed out abo\'e, the primitive fin-folds very soon 

 became strengthened by the appearance of a series of parallel 

 rods of cartilage set at right angles to the body, and at the line 



