536 



MANUAL OF ZOOLOGY 



median fin is usually present. The fore-limbs take the form 

 of flippers, with the digits covered over by a common integu- 

 ment and devoid of claws ; the hind-limbs are absent. The 

 mouth is very wide ; the nostrils are situated on the summit 



of the head, and the auditory 

 pinna is absent. Hairs are 

 completely absent, or are rep- 

 resented only by a few bristles 

 about the mouth. In the 

 whale-bone whales (Fig. 325) 

 the nostrils have two exter- 

 nal slit-like apertures ; in the 

 toothed whales, porpoises, and 

 dolphins, on the other hand, 

 the two nostrils unite to open 

 by a single crescentic valvular 

 aperture. 



In the Sirenia also the body 

 is fish-like, with a horizontal 

 caudal fin, the fore-limbs flip- 

 per-like, the hind-limbs absent, 

 and the integument almost 

 hairless. But the body is dis- 

 tinctly depressed, and the 

 head is by no means so large 

 in proportion as in the Ceta- 

 cea, and has a tumid truncated 

 muzzle, not far back from the extremity of which the nostrils 

 are situated. There is no dorsal fin. 



In the Ungulata vera the claws or nails of other mammals 

 are replaced by thick solid masses, the hoofs, investing the 

 ungual phalanges and bearing the weight of the body. The 

 number of digits is more or less reduced, and the limbs as a 



Fig. 325. — Sect! 

 baleen-plates, of 

 Owen.) 



ion of upper jaw, with 

 altsiioptera. (After 



