636 THE AMERICAN NATURALIST. [VoL. XXXIII. 
the southern boundary of the United States and Panama, of 
which four only extend north of this boundary. In 1877 and 
1878 the number recognized, respectively, by Allen and Alston, 
in their well-known papers on the American Sciuri, was 9 and 7. — 
Not only, however, was the material then available compara- 
tively scanty and poor in quality, but, as Mr. Nelson observes, 
these authors “were handicapped by the prevailing tendency 
of the time to lump species.” 
Under “ Notes on Distribution and Variation ” Mr. Nelson 
refers to the definiteness of the ranges of the various species, 
which is governed by the distribution of the different kinds of 
forest growth, which in turn is controlled by temperature and 
moisture. 
In his review of the “Subgenera of North-American Species” 
Mr. Nelson recognizes 10 subgenera, of which four are here first 
characterized, namely, Baiosciurus, Arzeosciurus, Otosciurus, 
and Hesperosciurus. Three of the 10 subgenera are practi- 
cally confined to that portion of North America north of 
Mexico (one entirely so, and two others extending south of the 
United States only into northern Lower California); two are 
properly South American, while the remaining five are common 
to the United States and Mexico. These groups are charac- 
terized by differences in size and proportions, together with 
differences in the shape of the skull and in dentition, and seem 
to form natural groups, though in general but slightly differen- 
tiated. By far the greater part of the tropical forms (10 species 
and 13 subspecies) belong to the single subgenus Ehinosciurus, 
which is unrepresented north of Mexico. 
According to Mr. Nelson: “ Tree squirrels occur in suitable 
places throughout Mexico and Central America, but the distri- 
bution of the various species depends largely upon the character 
of the forests. Thus Sciurus negligens is most abundant in the 
low dense forests of ebony, less than twenty-five feet high, on 
the hot coast plains, while its near relative, S. deppei, loves the 
shady depths of the humid tropical forests on the lower moun- 
tain slopes where the damp air produces an exuberant tree 
growth and an abundance of parasitic plants... . The larger 
species exist under even more varied conditions, since they 
