THE ALLIES OF THE SQUIRREL 415 



habit of feigning death when in danger of capture, hence the 

 expression, "playing possum. " 



Fossils from the rocks of the latter part of the Age of Rep- 

 tiles show that marsupials were widely distributed over the 

 earth at that time. Then came the separation of the Austra- 

 lian continent from the land to the north, isolating the mar- 

 supial fauna from the larger and more diversified land areas. 

 The Australian fauna evolved along various lines, produc- 

 ing herbivorous, insectivorous, and carnivorous forms which 

 resemble in outward appearance the members of the higher 

 orders of mammals, though they are in reality marsupial in 

 structure. Over the rest of the world, with the exception of 

 America, the marsupials were entirely destroyed and their 

 places taken by more highly specialized types. Marsupials 

 retained a foothold in South America owing to the absence 

 of overpowering enemies, and on account of their adaptation 

 to climatic and general environmental conditions. They 

 afford an illustration of discontinuous distribution. 



Sloths, Armadillos, and Allies. Both terrestrial and ar- 

 boreal animals are included in the order Edenta'ta (Lat. e, 

 " out " ; dens, " tooth "). Edentates have incompletely de- 

 veloped teeth, or if the teeth are well developed, they are 

 of simple structure ; true incisors are never present. Many 

 members of the order have a covering of scales formed from 

 the hardening of the skin. 



The armadillos (Fig. 218) are terrestrial American forms 

 which are protected by the scaly covering just referred to. 

 The tail and head are generally exposed, but the animals can 

 roll themselves into a ball, thus offering a hard surface in 

 every direction. 



The sloths of South and Central America are nocturnal, 

 arboreal animals ; their natural attitude during the day is 

 hanging from a branch back downward. The hair is gray, 

 but in some species it offers a lodgment for a green alga, a 



