SCIURID4—SCIURUS. 659 
or subspecific forms are permanently white below. The large number of 
synonyms that have arisen in consequence of this variability in color, together 
with the variability itself, render the recognition and characterization of the 
species exceedingly difficult. ‘The abundance of the material accessible for 
the study of the species occurring north of Mexico renders this part of the 
work comparatively easy, and demonstrates clearly the large amount of both 
individual and geographical variation one must be prepared to recognize, 
frequently at least, in the different specific representatives of this perplexing 
group. Some of the Mexican species seem to be fully as variable in colora- 
tion as any that are met with in the United States, so that color alone becomes 
a wholly unsafe guide for the determination of the species. On the other 
hand, Sciurus fossor, of the west coast of the United States, is as constant 
in its coloration as any Mammal with which I am acquainted, and the Brazilian 
form of Scturus @stuans, although widely distributed, presents only a small 
range of variation in either color or size. The large Squirrel of Western 
Brazil and the neighboring region to the westward (S. variabilis), though 
subject to considerable variation in color, is far less variable than some of the 
Mexican and North American ones. Three, at least, of the Central American 
forms also vary less than many of the others: these are Sczwrus gerrardi, 
S. tephrogaster, and S. @stuans var. rufjoniger, which range southward into 
the northern States of South America. 
The material at my command, though large in amount (probably con- 
siderably exceeding that ever before collectively examined by any previous 
investigator), is quite insufficient fora satisfactory study of the tropical forms, 
and the results arrived at are considered as open to future revision. Among 
-_ the numerous (some thirty or more) nominal species of authors, I have been 
able to recognize only nine or ten that seem to me valid, with two additional 
subspecies. These latter may possibly be entitled to specific rank, but I 
think that, on the whole, the number of species will, by future investigation, 
be still further reduced, rather than increased, and that I have erred in recog- 
nizing too many species rather than too few. In the majority of instances, 
I have been able to make the collocation of the nominal species with a good 
degree of confidence; in other cases, of course, only with much doubt; while 
two or three names I have been wholly unable to identify. One is doubtfully 
American, and one or two I have provisionally adopted as possibly valid 
species are yet unknown to me from specimens. 
In regard to the geographical distribution of the species, it is worthy 
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