Nor does the geographical distribution of the animals lead to any more definite conclusions. Certain families and even sub-families are of very wide range, the Muridæ and Sciuridæ especially being represented nearly all over the world, while the Hystricidæ and Leporidæ are also spread over very large areas, occurring in both hemispheres. Certain groups, such as the Sciuridæ of both sub-families, the Castoridæ, the Murine, and Arvicoline sub-families of Muridæ, the Leporidæ, and the Lagomyidæ, may be said to have a circumpolar distribution in the northern hemisphere, nearly allied and sometimes identical species being found in the more northern parts of both the Old and the New World, but mixed with other forms peculiar to the regions, especially as we advance southwards. On the other hand, the Myoxidæ are peculiar to the eastern hemisphere, as are also the Spalacidæ and the Dipodidæ (with the exception of _Zapus_, which is considered by Dr. Coues to form a distinct family), and all the sub-families of Muridæ, except those above mentioned as having a circumpolar range. The Myomorpha may in fact be looked upon as an Old World group, the Geomyidæ being the only exclusively American family; while the Hystricomorpha as a whole may be regarded as American, certain aberrant forms of the Octodontidæ inhabiting various parts of Africa and the Old World Porcupines being the sole representatives of that great section outside the western continent. Considering these facts, we may regard the Sciuromorpha and the Duplicidentata as originally polar types, or at all events as having an equal claim to an origin in the northern regions of either continent; while the Myomorpha, with their multitudinous forms spreading over all parts of the Old World, and having a much scantier representation in America, probably originated in the eastern hemisphere, and spread by a northern passage into the New World; and the Hystricomorpha would seem to have originated in South America, where they display the greatest variety of forms. W. S. DALLAS. [Illustration: GROUP OF SLOTHS (_Arctopithecus griseus_).] ORDER EDENTATA, OR BRUTA (ANIMALS WITHOUT FRONT TEETH). CHAPTER I. TARDIGRADA, OR SLOTHS.