238 



ORDER CETACEA ; GENERAL CHARACTERS. 



in water. But there is a marked difference in the arrangement 

 of the posterior instruments of propulsion ; for the pelvis and 

 hinder extremities are here entirely wanting (or, at least, only 

 rudiments of them can he discovered), and the tail is expanded 

 horizontally, so as to present a large surface for striking the 

 water. In some of the larger species, the breadth of this tail -fin 

 is 20 feet, and its surface not less than 100 square feet. The 

 force with which it strikes the water is enormous, the muscles by 

 which it is moved being of prodigious size. The greatest rapidity 

 of motion is produced, when its strokes are alternately upwards 

 and downwards; but the more ordinary progression of the 

 animals is accomplished by an oblique movement of the tail 

 downwards and laterally, first to one side and then to the other ; 

 much in the same manner as a boat is propelled by sculling. The 

 vertical motion of the tail is most employed, when the animal 

 desires to come to the surface to breathe. The paddles or 

 swimming paws are comparatively short ; but, as the enormous 



development of the 

 head in the true 

 Whales causes them to 

 be situated near the 

 centre of gravity of 

 the whole mass, they 

 act with considerable 

 power. They contain 

 the same bones as the 

 anterior extremities of 

 other Mammals ; but 

 they are more consoli- 

 dated together than in 

 any of the orders we have yet considered ; and there is but little 

 motion except at the shoulder-joint. They are completely 

 enveloped in a thick skin ; which prevents any trace of these 

 bones from being seen externally. 



209. The character of the skin in these animals is very pecu- 

 liar. In other animals in which there is a tendency to the 

 accumulation of oily matter, it is disposed as a layer of adipose 



FIG. 121 SWIMMING-PAW OF WHALK. 



