90 SCIURID, SCIURINZ. 
The family first to be considered of this order is that of the Squir- 
rels, and excepting the Australian region and the Island of Mada- 
gascar, these animals are found in nearly all the temperate and trop- 
ical regions of the world. They may be divided into two classes, 
the tree squirrels and the ground squirrels, with a kind of connecting 
link in the chipmunks of the genus Tamias, which, to a considerable 
extent, possess the habits of both. While the tree-squirrels are found 
in both temperate and tropical zones, the ground squirrels are dwel- 
lers of more northern climes, and some species are found even on the 
bleak shores of the Arctic Sea. In the tropics, however, the tree- 
squirrels attain their greatest diversity of coloration and highest 
development, and in the Oriental region they reach their greatest size 
and most brilliant hues. North America is perhaps the third on the 
list of those countries in which squirrels are found, being exceeded in 
number of species by the Indian and Ethiopian Regions. Europe 
and South America have comparatively few species of this family. 
Marmots, known usually as woodchucks or ground hogs, are the 
largest members of the Sciuride, but none are found within the 
limits of this work, and the little chipmunks are among the smallest. 
North America is probably the richest of all lands in ground squirrels, 
having a large number, varying greatly in size and coloration. 
Fam. 1. Seiuridz. Squirrels, Marmots, ete. 
Coues and Allen, Monographs of North American Rodents, U. S. 
Geol. Survey, 1877. 
E. W. Nelson, Review of the Squirrels of Mexico and Central 
America, Proc. Wash. Acad. Scien., 1, 1899, pp. 15-106. 
Tail without scales, cylindrical, bushy, hairs long; distinct postor- 
bital processes; infraorbital opening small; molars rooted, tubercular; 
first upper premolar small. 
Subfam. I. Sciurinee. 
KEY TO THE GENERA AND SUBGENERA. 
A. Upper incisors grooved. PAGE 
a. Size medium. Premolars, —............ Synthetosciurus gt 
B. Upper incisors not grooved. 
a. Size variable; tail flat, bushy, long. Skull 
short, broad; braincase more or _ less 
arched. Premolars, or —.................SCturus 93 
b. Size small, total length under 450 mm. 
a.’ Premolars, = == 
a./’ Superior outline of skull greatly curved. .Tamiasciurus 132 
