CLASS MAMMALIA 



437 



to form finlike flippers. It includes such gregarious forms as the seals, 

 walruses, and sea lions, which are often found collected in rookeries on 

 islands, particularly in the Arctic regions. The fur seal is very important 

 commercially. 



The order Rodentia (ro den' shi a; L., rodentis, gnawing) contains 

 mostly small animals characterized by the absence of canine teeth and 

 the great development of the incisors, which are used in gnawing. The 

 incisors continue to grow throughout life and the wearing away of the 

 soft dentine behind leaves the hard 

 enamel at the front of the tooth con- 

 stantly extended beyond the rest as a 

 sharp, cutting edge. This order in- 

 cludes the hares, rabbits, squirrels, rats, 

 porcupines, and beavers; it is 



mice, 



very rich in species, these numbering 



about one-third of all of the species of 



mammals. 



Edentata (e den ta' ta; L., edenta- 

 tus, rendered toothless) includes the 

 highly modified and decidedly ar- 

 chaic sloths, armadillos, and ant bears. 

 They are found mostly in South 

 America, although some species occur 

 as far north as Texas. Ants form a 

 large part of their diet. In spite of 

 the name Edentata, ant bears alone 

 are toothless; the others possess teeth, 

 but these lack enamel and are absent 

 in the front part of the jaws. The 

 sloths are interesting because they are '^^^ tetradactyla. A partly arboreal 



. . edentate, with prehensile tail, long 



distinctly arboreal animals, having a snout, small mouth, long and wormlike 

 habit of hanging from the underside of t^^^s^®' ''''^ '?\*f.*.^- ^* eats mainly 



^ . . . termites, and inhabits forests oi tropical 



branches and following them m this America. {Photographed from a speci- 

 position in locomotion (Fig. 334). The J/^J^^^/ University of Nebraska state 



armadillos (Fig. 332) have a well- 

 developed dermal skeleton consisting of bony plates covered with horny 

 scales in which hairs are embedded. When danger threatens, the animal 

 curls itself up in its shell and thus protects itself. 



445. Primates. — The lemurs, monkeys, apes, and man are primates. 

 Of these the lemurs are the most primitive and least manlike, having 

 many resemblances to the clawed mammals. In appearance they seem 

 to be intermediate between squirrels and monkeys. They are found 

 mostly in Madagascar but also in Africa and the Malay Archipelago. 

 The monkeys are divided into two types. The New World monkeys 



Fig. 333. — ^Lesser anteater, Taman- 



