652 MONOGRAPHS OF NORTH AMERICAN RODENTIA, 
as a rule, deep fulvous or rufous, unless displaced by melanism. In Southern 
Mexico and the States of Central America and Northern South America, the 
whole pelage often becomes wholly red,* or red varied with black. The S. 
estuans of Brazil also runs into ferruginous phases in the Central American 
States. In Mexico and Central America, the Sciuri exhibit a strong tendency 
to melanism, dusky or wholly black phases of coloration occurring with 
greater or less frequency in all of the species from this region known to me. 
The difficulty of distinguishing the species, especially merely by the 
study of museum specimens, is hence very great. While the Sciwri of North 
America have presented perplexing phases of coloration, those of the warmer 
parts of the continent do so to a far greater degree, several of the species 
presenting variations of color at one and the same locality greater even than 
does the highly variable Fox Squirrel of our Southern Atlantic and Gulf 
States. Of some of the species, no two specimens in series of large extent, 
from nearly the same locality, can be found that do not more or less widely 
differ in color. The widely distributed Sciurus estuans of South America 
is, however, remarkably constant in its coloration over a very extensive area, 
differing much less even than does the eastern form of Sciurus hudsonius, or 
any of the North American Sciuri, excepting perhaps S. fossor. Under these 
circumstances, it is surprising that more synonyms have not arisen. 
In addition to the purely geographical variation in color that the Squirrels, 
in common with other Mammals, present, there are other variations dependent 
upon season and age. Among the first are the presence or absence of ear- 
tufts in some of the Sczuri, the length and softness of the pelage, the variable 
degree of hairiness of the soles of the feet, and, in some species, differences 
of coloration. With differences of age occur differences in dentition, in the 
size, and even in respect to the presence of cranial crests and ridges for the 
attachment of muscles, and in other features. These points, as well as 
geographical variation in size and color, are treated at length in the descrip- 
tions of the several species and varieties. 
With these preliminary remarks, we pass now to a consideration of the 
genera, species, and varieties of the American Sciuride; no division of the 
family into subfamilies or higher groups, as previously stated, being requisite. 
“Dr. Coues’s studies of the variation of Putorius frenatus show a very instructive and parallel 
intensification of color to the southward, where the head becomes quite blackish, with obliteration of 
white stripes, and the salmon-color of the belly deepens to bright rusty or even orange red, 
