74 



ANIMAL LOCOMOTION. 



pally concerned in progression, these flapping about in the 

 water very much as the wings of a bird flap about in the air. 

 In the beaver, the tail is flattened from above downwards, as 

 in the foregoing mammals, but in swimming it is made to 



Fig. 35.— Skeleton of the Dugong. In this curious mammal the anterior 

 extremities are more developed than in the manatee and porpoise, and 

 resemble those found in the seal, sea-bear, and walrus. They are useful 

 in balancing and turning, the tail being the effective instrument of propul- 

 sion. The vertebral column closely resembles that of the fish, and allows 

 the tail to be lashed freely about in a vertical direction. Compare with 

 fig. 29, p. 65.— (After Dallas.) 



act upon the water laterally as in the fish. The tail of the 

 bird, which is also compressed from above downwards, can 

 be twisted obliquely, and when in this position may be made 

 to perform the office of a rudder. 



Swimming of the Seal, Sea-Bear, omd Walrus. — In the seal, 

 the anterior and posterior extremities are more perfect than 

 in the whale, porpoise, dugong, and manatee; the general 



Fig. 36 —The Seal {Plioca fcetida, Miill.), adapted principally for water. The 

 extremities are larger than in the porpoise and manatee. Compare with 

 figs. 33 and 34, p. 7^.— Original. 



form, however, and mode of progression (if the fact of its 

 occasionally swimming on its back be taken into account), is 

 essentially fish-like. 



