nalia: Classification 



76:5 



Skin very thick and nearly devoid of hair; only one pail' of mam- 

 mary organs, situated in extreme anterior (pectoral) region. Stomach 

 complex, resembling that of artiodactyls. Brain relatively small and 

 only slightly convoluted. 



Sirenians are the sea cows inhabiting the warmer coastal regions 

 of the Atlantic, Pacific, and Indian Oceans and adjoining bodies of 

 fresh water. They feed on aquatic vegetation. Length up to 8 or 10 

 feet. There are only two living Genera: 



Manatee (Trichechus or Manatus: Fig. 577): No incisors; only 

 six cervical vertebrae. Atlantic coastal regions of America and Africa. 



Dugong (Halicore): One pair of tusklike upper incisors; seven 

 cervical vertebrae, the usual number in mammals. Southwest Pacific 

 (Australia), Red Sea, Indian Ocean (east coast of Africa). 



A third recent Genus, Rhytina (Steller's sea cow), said to be 

 toothless when adult, inhabited the northern Pacific up to the eight- 

 eenth century, since when no observations of its occurrence have been 

 reported. 



Order 18: CETACEA. Aquatic; many of them of gigantic size (85 

 feet or more in length) and including the largest known mammals. 

 External form fishlike; no externally evident neck region; seven cer- 

 vical vertebrae much shortened and more or less fused together (Fig. 

 578). Caudal fin ("tail-flukes") horizontal instead of vertical as in 

 fishes; a single median dorsal fin usually present, but devoid of internal 

 skeleton. Pectoral limb short, broad, externally fmlike, but internally 

 having the skeletal parts of a typical pentadactyl limb; the four or five 



Fig. 577. Manatee, Manatus, (Courtesy, American Museum of Natural History, 

 New York.) 



