CHAPTER XXVIII 

 MANATEES AND DUGONGS ORDER SIRENIA 



THE purely aquatic Mammals known as manatees 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 somew r hat simi- 

 lar mode of life. 



Although the existing Sirenians resemble the Cetaceans in having 

 cs 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 conforma- 

 tion is very different. In the first place, although the body is somewhat Cetacean- 

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

 tail, it is markedly depressed, instead of being more or less compressed 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 inclosed 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 liv- 

 ing genera they consist of incisors and cheek-teeth. The structure of the cheek- 

 teeth is, however, very different in the two latter, 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, more- 

 over, furnished with a complete set of teeth, comparable to those of ordinary Mam- 

 mals. All the recent forms have horny plates on the palate and on the opposing 

 surface of the lower jaw. 



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