160 Bulletin American Museum of Natural History. [Vol. XXXIV, 
Size as a group character. 
Text Figures 1-12, pp. 162-164. 
South American squirrels present quite a range of difference in size, but 
the members of a closely related group of species agree so nearly in both 
size and proportions that often neither can be relied upon as diagnostic 
features, the range of individual variation in a dozen specimens of the same 
species, or subspecies, as the case may be, exceeding the average difference 
between allied forms. Mesosciwrus ' contains two subgenera, each of which 
comprises 12 forms. The two groups are not closely allied, either in size 
or coloration. The difference in average total length in the 12 forms of 
the first group is less than 20 mm., while the range in individual variation 
in total length ranges from 30 to 50 mm., or more, in each form. In the 
same group the average total length of the skull ranges from 54.5 to 57 mm. 
(a range of only 2.5 mm.), while the range of individual variation in each 
form runs from 3 to4mm. Taking the skull, which is the more trustworthy 
basis, and excluding three subspecies that are decidedly above the average, 
the average total length varies in the remaining 10 forms from 49 to 52 mm., 
a range of 3 mm., while the individual range in each form runs from 2 
to 4 mm. The same comparison could be carried through other groups 
with similar results. The point is, first, that closely related forms present 
a small average range of variation in size, inter se, and a very wide range 
of individual variation; second, that measurements, external or cranial, 
have little diagnostic value. 
The American squirrels fall into a number of superspecific groups in 
which difference in the size of the animal is a marked feature. The smallest 
are the pygmy squirrels (Sciwrillus and Microscturus) of the Guianas and 
Andean regions respectively, with a total length of about 225 to 260 mm., 
and the size increases by steps or stages to the giant squirrels of the Amazonas 
with an average length of about 575 mm., with individuals exceeding 600 
mm. The size of the skull varies, par? passu, from less than 30 to more 
than 60 mm., the giant squirrels being cubically about eight times larger 
than the pygmy squirrels. The feature of size is always correlated with © 
other differences, and should seemingly carry weight as a diagnostic charac- 
ter. It is presented graphically in accompanying illustrations. 
1 For diagnosis see postea, p. 182. 
