_M. brunneus, nigro lavatus; rostro ad apicem, labiis, mento, gulâ, pectore, abdomineque albis; naso suprà nigrescente; auribus mediocribus rotundatis; corpore crasso; caudâ capite corporeque breviore, pilis nigricantibus, subtùs albescentibus prope basin, vestitâ; artubus pedibusque grisescentibus; vellere longo, molli; unguibus longis._ DESCRIPTION.—Body stout; head large; tail nearly as long as the head and body; inner toe of the fore foot with a distinct, pointed claw; claws rather large, those of the fore feet but slightly arched. Fur rather long, and moderately soft; general tint of the upper parts of the body, brown, of the sides of the head and body, grayish, but with a yellow wash; the lower part of the sides of the body and of the cheeks, the tip of the muzzle, and the whole of the under parts, white; feet dirty white; ears densely clothed with short hairs, those on the inner side chiefly of an ashy-brown colour, and those on the outer side dusky; the hairs of the back are of a deep lead colour at the base, black at the tip, and annulated with yellow near the tip; the longer hairs, which are thickly interspersed, are totally black; on the under parts of the body the hairs are gray at the base, and broadly tipped with white; the upper surface of the muzzle is blackish; the moustaches are black; the incisors are yellow. In. Lines. Length from nose to the root of tail 6 9 of tail 5 4 from nose to ears 1 8 of tarsus (claws included) 1 6 of ear 0 7 Habitat, Maldonado, La Plata (_June_.) This species is about the size of _Mus Rattus_, but is stouter in its proportions; as in _Mus nasutus_, the thumb is furnished with a pointed claw. The molars of the lower jaw are figured in Plate 34, fig. 11, _a_. “This rat was caught in so wet a place amongst the flags bordering a lake, that it must certainly be partly aquatic in its habits.”—D. 21. MUS BRAZILIENSIS. PLATE XIX. Rat du Brézil, _Geoff._ _M. suprà fuscus fulvo lavatus; lateribus capitis corporisque æquè ac abdomine auratis; gulâ pectoreque albis; pedibus pilis sordidè flavis tectis; auribus parvulis; caudâ caput corpusque ferè æquante; vellere longo, molli._