720 ELEPHANTS xxix. 9 



9. Order Sirenia 



The sea-cows are herbivorous creatures, living along the coasts 

 and in rivers, and highly adapted to aquatic life. There is little doubt, 

 however, that they have reached this condition by modification of a 

 basic ungulate type of organization, probably not very different from 

 that of the early proboscidians. The two modern forms, the manatee 

 of the Atlantic (Fig. 478) and dugong of the Pacific and Indian oceans, 

 are different in many respects and represent lines that have been 

 separate for a long time, probably since the Eocene. Manatus ( = 

 Trichechus) has three species on the Atlantic coasts and in the rivers of 



Fig. 478. Manatee, Manatus. (From photographs.) 



Africa and America. Dugong (= Halicore) is a purely marine animal 

 extending from the Red Sea throughout the Indian Ocean to Formosa 

 and Australia. Rhytina (Steller's sea-cow) was an Arctic form that 

 became extinct in the eighteenth century. 



Sea-cows have a 'streamlined' body-form, with few hairs and thick 

 'blubber'. There are no hind-limbs and the pelvic girdle remains only 

 as small rods to which the corpus cavernosum is attached in the 

 male. The fore-limbs are large, the digits joined to form paddles, 

 with a full pentadactyl structure and no hyperphalangy or hyper- 

 dactyly. The caudal vertebrae are well developed and swimming is 

 effected by the body and tail, the latter carrying a terminal horizontal 

 fin. The vertebrae articulate with each other by flat surfaces, as in 

 other aquatic forms, but there are zygapophyses, and the whole 

 column is not quite reduced to the condition of a simple compression 

 strut. The bones have a characteristic structure (pachyostosis), prob- 

 ably produced by lack of stressing. The manatee has only six cervical 

 vertebrae. The ribs are round and the diaphragm is oblique, as in ele- 

 phants and whales, allowing the lungs to reach far back. Respiration 

 is probably mainly by means of the barrel-like ribs. The lungs contain 



