Specimen from Buenos Ayres Maldonado. Valparaiso. In. Lines. In. Lines. In. Lines. Length from nose to root of tail 9 9 9 3 8 6 of tail Imperfect 6 0 6 6 of tarsus 1 7 1 7 1 7 East East Keeling Falkland. Falkland. Island. In. Lines. In. Lines. In. Lines. Length from nose to root of tail 8 9 9 0 8 3 of tail Imperfect 6 0 6 6 of tarsus 1 7 1 7 1 7 Upon comparing the skull of the Valparaiso variety with that of a British specimen of _Mus decumanus_, I could perceive no difference. A skull from West Falkland did not differ, neither did the dentition of the Keeling Island specimen above noticed. A perfect specimen of this last I have not had an opportunity of examining. 2. MUS (DECUMANUS _var._ ?) MAURUS. Mus maurus, _Waterh._ in Proceedings of the Zoological Society of London, for February, 1837, p. 20. _M. pilis suprà purpurescenti-nigris; subtùs plumbeis; auribus parvulis, pallidè fuscis: caudâ corpus ferè æquante._ DESCRIPTION.—The character of the fur of this animal nearly resembles that of _Mus decumanus_; it is, however, of a harsher nature: the general colour of the upper parts and sides of the body is purple-black, arising from the longest hairs being of this colour, and likewise the tips of those which are next in length; the latter, however, excepting at the tip, are white, and this white is not entirely hidden, even when the hairs are in their ordinary position: on the head the hairs assume a brownish hue, and are tolerably uniform: the limbs, and under parts of the body, are of a deep gray colour, with a faint purple-brown wash: the under fur is gray: the ears are small, of a brown-white, or very pale brown colour, and furnished with minute brown hairs: the small, scattered, bristly hairs of the tail are of an uniform brownish black colour. The hairs of the moustaches are black at the base, and grayish at the apex. In. Lines. Length from nose to root of tail 11 3 of tail 7 6 of tarsus 1 8 of ear 0 6¼ from nose to ear 2 2 Habitat, Maldonado, La Plata, (_June_.)