298 Bulletin American Museum of Natural History. [Vol. XXXIV, 
Throughout this system squirrels are found in considerable variety as far 
south as about to the northern border of Chile, beyond which they appear 
to have failed to penetrate, their most southern point in the western half of 
the continent being southern Bolivia. 
On the eastern side of the continent, south of the Venezuelan Ilanos, 
they occupy the Guiana Highlands and the forested parts of the Brazilian 
Highlands south to southern Brazil, and extend thence westward in the 
wooded parts of the drainage areas of the Orinoco and Amazon to the 
eastern base of the Andes. But these several widely different physiographic 
areas are each occupied by special types of squirrel, none of the groups 
recognized as generic being common to any portion of both the eastern and 
ih | 
Ae five 
Fig. 22. 1, Microscturus; 2, Sciurillus.1 
western divisions of the continent. The distribution of the genera and 
species may be briefly stated as follows, premising that the Caribbean border 
of the continent is to be regarded as part of the western division. 
Two very obvious characteristics of the sciurids of South America, the 
number of premolars and the number of mamme, as noted in the early part 
of this paper (pp. 158, 159), present great constancy throughout vast geo- 
graphic areas. All the groups with 6 mamme are restricted (with the one 
1The accompanying distribution. maps (Figs. 22-25) are of course to be taken as only 
approximate, and largely hypothetical so far as the exact boundaries of the areas are con- 
cerned, being diagrammatic expressions of our present knowledge of the subject. 
