CHAPTER XXVIII. 



Manatis and Dugongs, — Order Sirenia. 



The purely aquatic mammals known as manatis and dugongs, together with the 



northern sea-cow, which has become extinct within the last century and a half, 



constitute an order by themselves, and may be collectively known as Sirenians. 



Although they are as well fitted for an aquatic life as the Cetaceans, these animals 



have no sort of relationship with the members of that order, and have evidently 



been derived quite independently from terrestrial mammals. Such resemblances 



as do exist between Sirenians and Cetaceans are entirely of an adaptive nature, and 



have been produced merely by the two groups of animals leading a somewhat 



similar mode of life. 



Although the existing Sirenians resemble the Cetaceans in having 



Characteristics. . D . , ° 



their fore-limbs converted into flippers, and having lost all traces of 



the hind -limbs, while the tail is converted into a horizontally-expanded rudder- 

 like organ, comparable to the flukes of the whales and dolphins, their general 

 conformation is very different. In the first place, although the body is somewhat 

 cetacean-like, without any well-defined neck and with no distinction between 

 trunk and tail, it is markedly depressed, instead of being more or less com- 

 pressed from side to side. Then, again, the head departs but little from the 

 ordinary mammalian type, being comparatively small in proportion to the body, 

 with the summit rounded, and the nostrils, which are double and capable of 

 being closed at will by valve-like flaps, placed above the extremity of the 

 abruptly-truncated muzzle. The back-fin, so commonly present in the Cetaceans, 

 is totally wanting. In the flippers, although the whole of the toes are enclosed 

 in a paddle-shaped mass of integument, traces of nails are still in some cases 

 retained. The eyes are small, with imperfectly-developed lids, and the minute 

 aperture of the ear is unprovided with any external conch. The mouth is small, 

 with thick, fleshy lips, upon which grow a number of bristly hairs, which 

 persist throughout life. The skin is thick, and either finely wrinkled or rugged 

 and bark-like, sometimes with fine hairs thinly distributed upon it. The female 

 has a single pair of teats placed on the breast. The teeth are very variable, being 

 totally wanting in the northern sea-cow, while in the other two living genera they 

 consist of incisors and cheek-teeth. The structure of the cheek-teeth is, however, 

 very different in the two Litter, and in one of them their number is much greater 

 than among less aberrant mammals. The living forms have been recently discovered 

 to possess rudimental milk-teeth, and in some extinct species such teeth were well 

 developed. Certain extinct members of the order were, moreover, furnished with 

 a complete set of teeth, comparable to those of ordinary mammals. All the recent 



