DESCRIPTION.—Ears large; tail rather shorter than the head and body taken together; tarsi slender, and moderately long; fur long and very soft; general tint of the upper parts of head and body grayish, washed with brownish yellow; on the sides of the body a palish yellow tint prevails; feet, chin, throat, and under parts of body pure white; tail rather sparingly clothed with hairs, those on the apical portion rather long, and forming a slight pencil at the tip; on the upper side and at the tip of the tail the hairs are brown, on the under side they are dirty white; the ears are very sparingly clothed with minute brownish yellow hairs internally; externally, on the fore part, the hairs are rather longer and of a brown colour; the upper incisors are orange, and the lower incisors are yellow; the hairs of the moustaches are long, and of a black colour; the hairs of the back are deep gray at the base, brownish at the tip, and annulated with pale brownish yellow near the tip; the longer hairs are brown; the hairs of the belly are white externally, and gray at the base; on the throat the hairs are white to the root. In. Lines. Length from nose to root of tail 6 8 of tail 5 6 from nose to ear 1 4½ of tarsus (claws included) 1 2½ of ear 0 8 Habitat, Northern Patagonia (_August_.) The molars of the upper jaw are figured in Plate 34, fig. 15, _a_, and those of the lower jaw, fig. 15, _b_. “Inhabits the dry gravelly plain, bordering the Rio Negro.”—D. 24. MUS XANTHOPYGUS. PLATE XXII. Mus xanthopygus, _Waterh._, Proceedings of the Zoological Society of London for February 1837, p. 28. _M. suprà pallidè brunneus flavo-lavatus, ad latera flavescens, subtùs albus; capite griscescente; natibus flavis; pedibus albis; auribus majusculis pilis, albis et flavis intermixtis obsitis; caudâ quoad longitudinem, corpus ferè æquante, suprà nigricante, subtùs albâ; vellere longo et molli; mystacibus perlongis albescentibus, ad basin nigris._