MAMMALS 407 



are of course not pigs, and they do not come 

 from Guinea ; the original intention was to say 

 Guiana pig. They constitute a peculiar 

 South American genus, and should be known 

 as cavies (Cavia). The Norway rat and house 

 mouse are of Old World origin, and have been 

 introduced into America by man. The native 

 American rats and mice belong to different 

 genera, although the genera Castor (beavers), 

 Marmota (marmots and woodchucks), Sciurus 

 (squirrels), and some genera of voles are com- 

 mon to the New and Old Worlds. The prairie 

 dog (Cynomys, meaning "dog mouse") is 

 peculiar to North America ; it is essentially a 

 squirrel modified for life on the treeless plains. 

 The rabbits, hares, and pikas are usually 

 placed with the rodents, from which they differ 

 by having two pairs of incisor teeth in the upper 

 jaw. Anatomical evidence has lately been 

 presented, which seems to show that these ani- 

 mals constitute a group distinct from the true 

 rodents and of quite independent evolution. 

 (e) Edentata. Sloths, anteaters, and armadillos, sloths, ant 

 all American. The name of the order means 

 "toothless," and is accurate as applied to the 

 anteaters, but not to the others. The ground 

 sloths, now all extinct, but living within com- 

 paratively recent times, were immense ani- 

 mals, comparable in size with elephants and 

 rhinoceroses. There is evidence that a species 

 of these animals was contemporaneous with 

 man in South America, and pieces of its skin, 

 with hair attached, have been discovered in 

 a cave. The armadillos are remarkable for 



