This mouse is considerably larger than _Mus Musculus_, and the great length and density of its fur, causes it to appear much stouter in its proportions; its colouring is darker, the tips of the hairs being much more narrowly annulated with yellow than in that species. The very small size of the ears will serve to distinguish the present animal from its congeners—_Mus longipilis_, _M. Renggeri_, _M. arenicola_, &c. The molar teeth of the upper jaw are figured in Plate 34. fig. 8, _a_; and fig. 8, _b_, represents the middle and last molars of the lower jaw. “Inhabited a very small island, covered with thick forest, in the central part of the Chonos Archipelago.”—D. A mouse obtained on the islets adjoining the east coast of Chiloe (where Mr. Darwin says it was common) differs from the above in being a little smaller, the tail is rather longer, and the ears are a trifle larger. In the feet, claws, colouring and character of the fur it agrees, and likewise in the pale colour and slenderness of the incisors. Its dimensions are as follows:— In. Lines. Length from nose to root of tail 4 0 of tail 3 0 of tarsus (claws included) 0 10 from nose to ear 0 10½ of ear 0 4 I have not the means of satisfying myself whether this be a distinct species or not; but I think it is not. “The nature of the country where this specimen was procured is nearly the same as in that part of the Chonos Archipelago, 150 miles to the south, where the first was obtained.” D. 14. MUS RENGGERI. PLATE XV.—Fig. 1. Mus olivaceus, _Waterh._, Proceedings of the Zoological Society of London, for February 1838, p. 16. _M. corpore suprà subolivaceo, subtùs cinerescente; auribus mediocribus, rotundatis, pills parvulis fuscescentibus obsitis; caudâ corpore breviore, pilosâ, suprà fuscâ subtùs albescente; pedibus pilis fuscescentibus tectis._