INTRODUCTION XV 



at their tips, but in many others the front rays of the dorsal 

 and anal fins and the outer rays of the pectorals and pelvics 

 are transformed into sharply pointed, stiff spines, sometimes 

 with saw-like edges. Such spines provide useful offensive or 

 defensive weapons. 



Dorset o, StccA /iJt- 



TYyyoej- 



-Topography of a Cetacean. 



Fig. 3. — Flipper of a Sperm Whale. 



In the cetaceans the hinder end of the body is flattened out 

 into flukes (Fig. 2). These fleshy extensions are set in a 

 horizontal plane, whereas the caudal fin of a fish is always set 

 in a vertical plane ; this position of the tail provides a ready 

 means of distinguishing any fish from any cetacean. On the 

 middle of the back of cetaceans there is very frequently a 

 fleshy eminence, the dorsal or back fin, which may be triangular 



