The very peculiar structure from which the Marsupial animals derive their name has been regarded by almost every naturalist who has written on the subject as so essential a deviation from the common type, that, setting aside all considerations of form or habits, and regardless even of those technical characters on which so much reliance is usually placed, they have for the most part agreed in uniting under the same family designation every animal in which it occurred. This peculiarity consists in a folding or doubling of the skin and its appendages beneath the lower part of the belly in the females, in such a manner as to form an open pouch or bag, in which the young are contained from a very early period, in which the process of suckling takes place, and in which, even for some time after they have acquired sufficient size and strength to leave it, the little ones continue to take refuge. But the presence of this one anomalous characteristic is accompanied by so many striking discrepancies in other parts, that, limited as this tribe is in number, most of the principal forms of Mammalia find analogous representations among its groups. Thus the Opossums exhibit characters in some measure intermediate between the Quadrumana and the Carnivora, to which latter the Dasyuri, another Marsupial group, closely resembling the Civets in form and habits, approach very nearly; while the herbivorous races of the tribe might occupy a station between the Rodent and Ruminant Orders, with each of which they exhibit various degrees of relationship. This want of uniformity in the essential parts of their organization necessarily gives rise to much difficulty in determining their position in the system. The mode of classification now most generally followed is perhaps, under all the circumstances, the best that could at the present moment be adopted; although it must be owned that the purely herbivorous species arrange themselves with a very ill grace under a subdivision of the order Carnivora. Placed, however, as they are at the end of that order, and immediately before the Rodentia, the regular gradations from the type of the former to that of the latter, which occur in their different groups, become most distinctly manifest.