SIRENIA. 503 



as in the true Whales, and is not so gradually prolonged into 

 the body as it is in the latter. There may be only six cervical 

 vertebrae. The teats are two in number and are " thoracic," 

 > /. e., are placed on the chest. There are no clavicles, and 

 the digits have no more than three phalanges each. The testes 

 are retained throughout life within the abdomen, but vesiculae 

 seminales are present. The animal is diphyodont, the perma- 

 nent teeth consisting of molars with flattened crowns adapted 

 for bruising vegetable food, and incisors which are present in 

 the young animal, at any rate. In the extinct Rhytina it does 

 not appear that there were any incisor teeth. 



Fig. 192. Sirenia. Dugong (Halicore). 



The only existing Sirenia are the Manatees (Manatus) and 

 the Dugongs (Halicore), often spoken of collectively as " sea- 

 cows," and forming the family of the Manatidce. 



The Manatees are characterised by the possession of numer- 

 g g 



ous molar teeth (o~o), and of two small upper incisors, which 



are wanting in the adult. The tail-fin is oblong or oval in 

 shape, and the anterior limbs are furnished with nails to the 

 four outer digits. They occur on the east coast of North 

 America, especially in the Gulf of Mexico, and another species 

 is found on the west coast of Africa. They are generally found 

 in considerable numbers about the mouths of rivers and estu- 

 aries, and they appear to live entirely upon sea-weeds, aquatic 

 plants, or the littoral vegetation. They are large awkward 

 animals, attaining a length of from eight to ten feet as a rule, 

 but sometimes growing to a length of nearly twenty feet. 



The Dugongs (Halicore, fig. 192) have -^ molar teeth in 



2 2 



the young condition, but only - when old. Inferior incis- 

 ors are present in the young animal, but are wanting in the 

 adult. The upper jaw carries two permanent incisors, which 



