The profound and extensive studies upon the American Squirrels by Allen 1) and Alston 2), have made this group, we must confess it, one of the best known among the Mammals. At the same time however the named authors have shown that it is almost an impossibility to give a good and exact diagnosis of an American Squirrel, because the specimens of a given species differ, so to say, infinitely in the hairiness of the ears, which are tufted or not tufted, in the length and strength of the body, a. s. o. The difficulty increases as these variations are to be found not constantly in individuals from different localities, but as they often are present in individuals from the same locality and gathered on the same day or in the same season. And this is a rule with only a few exceptions. Here more than ever the accurate knowledge of the specimens is the knowledge of the species. Therefore I abstain from diagnoses of the several species, which moreover sufficiently have been given by Allen and Alston, but on the other hand I will endeavor to describe in terms as short as possible every specimen, which shows an aberrant or remarkable variation in its mode of coloration or otherwise.