DESCRIPTION.—General colour very pale yellowish brown, a tint produced by the admixture of black and pale fawn colour; the hairs of the ordinary fur being of the latter tint near the apex, and dusky at the apex, whilst the longer hairs are black. The feet, tail, under parts of the body and the sides of the muzzle, are pure white. All the hairs of the body, (which are soft, and of moderate length), are deep gray at the base. The ears are of moderate size, well clothed with hairs, of which those on the inner side are yellowish, and those on the outer, are brown on the anterior part, and white on the posterior. A small tuft of white hairs springs from the neck immediately behind the ears; this tuft is hidden when the ears are folded back. The tail is slender and short, (being not quite equal to the body in length) of a pale flesh-colour, and sparingly furnished with minute white hairs. The feet are very small and slender, and the naked parts are of a pale flesh-colour. The sole of the foot is covered with hairs; the toes beneath, and the tubercles (which are as in _Mus Musculus_), however, are naked. The hairs of the moustaches are of moderate length, and of a blackish colour, some of them, however, are grayish white. In. Lines. Length from nose to root of tail 2 10 of tail 1 7 from nose to eye 0 4⅓ from nose to ear 0 8¼ of tarsus (claws included) 0 6½ of ear 0 4¼ Habitat, Bahia Blanca, (_September_.) This species slightly exceeds the harvest mouse (_Mus messorius_) in size, its ears are considerably larger in proportion, and the tail is shorter. Compared with the common mouse (_Mus Musculus_) it is smaller, the tail is more slender, and shorter, and the feet are likewise more slender and proportionately much smaller; the ears are more distinctly clothed with hairs. The principal points of distinction between this and the two preceding species are pointed out in the account of _M. elegans_.