164 



LAND MAMMALS IN THE WESTERN HEMISPHERE 



Fig. 77. — Grey Squirrel {Sciurufs curolint'ntii.s). 

 — By permission of the N. Y. Zoolog. Soc. 



prairie-dogs {Cynomys), the white-footed mice (Sitomys), 

 wood-rats {Neotoma) and one genus of pocket-gophers {Thom- 

 omys) are chiefly Sonoran, but have Boreal representatives. 



The flying squirrels 



{Sciuropterus), true 

 squirrels (Sciurus), 

 ground-squirrels {Sper- 

 mophilus), rabbits (Le- 

 pus), wolves (Canis) 

 and otters (Lutra) have 

 a very wide range 

 through both the Bo- 

 I'eal and Sonoran, but 

 have many more spe- 

 cies in the latter region. 

 The Sonoran region 

 may be divided into 

 the upper and lower Sonoran zones, which are demarcated 

 by temperature and are of transcontinental extent. Each of 

 these zones may, in turn, be subdivided into arid and humid 

 provinces, but our purpose does not necessitate entering into 

 such refinements. 



The Neotropical, which is the only region of the Neogaeic 

 realm, comprises the West Indian islands, all of Central 

 and South America and the lowlands of Mexico, extending 

 a short distance into southeastern Texas. Of its four sub- 

 regions, the most typical is (1) the Brazilian, which includes 

 not only Brazil, but all of South America east of the Andes and 

 as far south as Paraguay, and is a vast area of tropical forests. 

 (2) The Chilian subregion takes in the west coast, the high 

 Andes and the southern end of the continent, south of the 

 Brazilian subregion ; it is a country chiefly of open plains and 

 high mountains, and a few deserts, of which South America 

 has less than any other continent, except Europe, which has 

 none. (3) The Central American subregion reaches from the 



