652 MOxoGRArns or noktii American eodentia. 



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. 

 (Bstuans 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 Sciuri 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 he found that do not more or less widely 

 differ in color. The widely distributed Sciurus cestuans 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 Sciuri, 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 Sciuridce ; no division of the 

 family into subfamilies or higher groups, as previously stated, being requisite. 



* Dr. Coues's studies of the variation of Putorius fremitus 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 bully deepens to bright rusty or even orange red. 



