In. Lines. Length from nose to root of tail 3 6[21] of tail 2 1 from nose to ear 1 1 of tarsus (claws included) 0 9½ of ear 0 4 Habitat, Santa Cruz and Port Desire, (_December_.) “Very common in long dry grass in the valleys of Port Desire.”—D. The skull is figured in Plate 33, fig. 5, _c_. Fig. 5, _a._ represents the molars of the upper jaw; fig. 5, _b._ those of the under jaw, and fig. 5, _d._ represents the posterior molar of the under jaw when more worn. It was with some hesitation that I described this as a distinct species in the Society’s Proceedings. I have now re-examined the specimens, and still am unable to satisfy myself whether they are varieties of _Mus xanthorhinus_ or not. Both of _Mus canescens_ and of _Mus xanthorhinus_, I have before me what I imagine to be an adult and a young specimen. The adult and the young of _M. xanthorhinus_ agree in being of a _yellowish brown_ colour, and in having the muzzle and tarsi deep yellow; both specimens of _Mus canescens_ are of a _gray_ colour, with an indistinct yellow wash, the muzzle and tarsi being tinted with yellow, as in _M. xanthorhinus_. Besides this difference in tint, which, perhaps, is unimportant, _M. canescens_ differs from _M. xanthorhinus_ in having the head larger, the tail rather longer, and the fur less soft. The specimens of this animal are both from Patagonia; one of the specimens of _Mus xanthorhinus_ was brought by Mr. Darwin from Terra del Fuego; and as the other formed part of Captain King’s collection, it in all probability came from the same locality. As I only possess one skull, I cannot speak with certainty as regards the size of the head; the difference, however, in the stuffed specimens is considerable, and it is strange that each of the pairs should agree so perfectly, supposing the difference to be the work of the stuffer’s hands. 18. MUS LONGIPILIS. PLATE XVI. Mus longipilis, _Waterh._, Proceedings of the Zoological Society of London for February 1837, p. 16. _M. suprà obscurè griseus, flavo lavatus; subtùs griseus; pedibus fuscis, unguibus longiusculis; auribus mediocribus; caudâ corpore breviore, suprà nigrescente, subtùs fuscescente; rhinario sub-producto: vellere longissimo, molli._