700 MAMMALIA [CH. 



and crushing, and front teeth sometimes are found developed as 

 tusks. There is no such snout as is found in Whales, but there are 

 large movable lips by means of which food is seized (Fig. 351). The 

 teats are placed on the breast as in Bats, and the mother when 

 nursing supports the young with its head above water by means of 

 the flipper which is more flexible than the flipper of a Whale. It 

 has been suggested that the tails of Mermaids may have been 

 suggested to sailors by the sight of these strange mothers holding 

 their pups on their arms. There are two genera: (1) Manatus, the 

 Manatee found in the warmer parts of the coastal waters of the 

 Atlantic and in the estuaries of its rivers both in America and 

 Africa, and (2) the Dugong, Halicore, found all around the coasts 

 of the Indian Ocean and round Australia where it is fished for and 

 eaten. Until 1768 a third species, Rhytina, stelleri, of great size, 

 20 25 ft. long, inhabited some islands in the Behring Sea. It 

 had no teeth, their place being supplied by horny plates on the 

 gums. This species was exterminated by Russian hunters. Re- 

 mains of extinct species of Sirenia with front teeth and vestigial 

 hind-limbs have been discovered and the structure of these renders 

 it probable that Sirenia are descended from some primitive Ungulate 

 allied to the Eocene ancestor of Elephants. 



Order VIII. Rodentia. 



The Rodentia or Gnawers (Lat. rodo, to gnaw) are another of the 

 main divisions of the Mammalia and include our Rabbits, Hares, 

 Squirrels, Rats and Mice, besides the Porcupine, Beaver, Guinea-pig 

 and many other foreign species. These are all sharply marked off 

 from other mammals by the structure of their teeth. The incisors, 

 of which there are typically only one pair in each jaw, are chisel- 

 shaped and covered with hard enamel on their outer sides only. 

 They constantly grow and are only kept down to proper size by 

 continual gnawing and rubbing against each other (Fig. 352). If 

 one of the teeth is destroyed the opposite one grows until it may 

 pierce the other jaw, prevent the mouth from being opened, and thus 

 starve the animal to death. There are no canines, so that there 

 is a great space or diastema between the front teeth and back 

 teeth. The claws are always blunt and nail-like, and walking is 

 done on the last joints of fingers and toes, not as in the case of the 

 Ungulates on the points of the nails (Fig. 322). Our English 

 Rodents are the Hares and Rabbits, LEPORIDAE ; the Squirrels, 



