A rare constancy in color of the fur in some species, a nearly endless variety in other species and a very peculiar geographical distribution render the study of this beautiful group of Marsupials very difficult but at the same time highly interesting and attractive. A thourough study based upon large and well selected series from the most different and distant localities is the only way to surmount the very difficulties. No wonder therefore that so many naturalists have failed in their efforts to recognize and circumscribe the species composing this group. The Cuscus-species have half or more than half the prehensile tail destitute of hair and the second and third toes of the hind feet united in a common integument very nearly to the extremity. These two characters combined distinguish them from all the other Mammals.