The skull of this animal, is rather shorter and broader than that of _Mus Musculus_, the upper surface is more arched, the zygomatic arch is much more slender, and the nasal bones are rather broader. In the convexity of the upper surface, and the slenderness of the zygomatic arch, this skull very nearly resembles that of _M. gracilipes_; this latter, however, has the zygomatic arch more convex, projecting more suddenly on the anterior part, and the interparietal bone smaller. Length of skull 10 lines, width 5½, length of nasal bones 4 lines, distance between the outer side of the incisors, of the upper jaw, and the first molar 2⅞ lines. See Plate 34, fig. 3. _a_. The dentition is figured in Plate 34, figs. 3. _b_ and _c_. This mouse is rather less than _M. Musculus_, the tail is much shorter in proportion, the fur is longer and softer, and the ears are more distinctly clothed with hair. The pale and delicate yellow colour of the upper parts of the body, and the pure white of the under parts, renders the present species conspicuous amongst its congeners. I may further remark that the white colour which in the Muridæ (when it occurs) is usually confined to the under part of the body, or extends but slightly on the sides, is in the present animal extended considerably on the sides of the body, and occupies an equal portion with the yellow of the upper parts. The name _bimaculatus_ is applied to this animal on account of the two conspicuous white patches, which are situated behind the ears. In affinity as well as in appearance it most nearly approaches to _Mus gracilipes_ and _M. elegans_; with no other species of the genus _Mus_, here described, can it be confounded, since these only have the tarsus hairy beneath. The principal points of distinction between the present animal and _Mus elegans_, are noticed in the account of that species. “This mouse, when alive, had a very elegant appearance. A countryman, who brought it me, found six of them living together in one burrow.”—D. MUS GRACILIPES. PLATE XI. Mus gracilipes, _Waterh._, Proceedings of the Zoological Society of London, for February 1837, p. 19. _M. suprà flavo-lavatus; pilis pone aurem utramque, labiis, corporeque subtùs, albis; pedibus parvulis, gracilibus, carneis, suprà et ad calcem pilis albis tectis; caudâ gracili, pilis albis instructâ, quoad longitudinem corpus ferè æquante; auribus mediocribus; vellere mediocri et molli, pilis omnibus ad basin plumbeis._