DESCRIPTION.—Form stout, ears rather small, tail nearly equal to the body in length, fur very long and moderately soft, general colour of the upper parts of head and body, brown; of the sides of the body grayish, faintly washed with yellow, of the under parts grayish white, faintly tinted with yellow; hair covering the upper surface of the feet dirty white; on the tarsus there is a very slight yellow tint; ears well clothed with hairs, those on the inner side chiefly of a yellow colour; tail above, dusky brown; beneath dirty white: hairs of moustaches black at the base and grayish at the apex; incisors pale yellow: hairs of the back deep gray at the base, annulated with brownish yellow near the apex, and dusky at the apex; longer hairs dusky black; hairs of the belly deep gray at the base and broadly tipped with yellowish white. In. Lines. Length from nose to root of tail 6 0 of tail 3 8 from nose to ear 1 4 of tarsus (claws included) 1 0¾ of ear 0 6 Habitat, Santa Cruz, Patagonia, (_April_.) The molars of the upper jaw are figured in Plate 34, fig. 13, _a_, and those of the lower jaw, fig. 13, _b_. “Caught in the interior plains of Patagonia in lat. 50°, near the banks of the Santa Cruz.”—D. 23. MUS GRISEO-FLAVUS. PLATE XXI. Mus griseo-flavus, _Waterh._, Proceedings of the Zoological Society of London for February 1837, p. 28. _M. suprà griseus flavo-lavatus, ad latera flavus, subtùs albus; pedibus albis; auribus magnis et ferè nudis; caudâ caput corpusque ferè æquante, suprà fusco-nigricante, subtùs albâ; vellere longo, molli._