_Ab. suprà grisea, levitèr ochraceo lavata; abdomine gulâque albescenti-griseis; pedibus sordidè albis; auribus amplis, ad marginem posticum distinctè emarginatis; caudâ corpore multò breviore, et nigrescente._ DESCRIPTION.—Ears large; tail considerably shorter than the body; fur extremely soft; general colour gray faintly washed with yellow; under parts of the body grayish white; feet dirty white; tail dusky, paler beneath at the base: the ears are large, distinctly emarginated behind, and appear to be almost naked, but, upon close examination, long and extremely fine hairs may be observed. All the fur is gray at the base; the hairs of the moustaches are numerous and very long, those nearest the mouth are white, the others are black at the base and grayish beyond. The incisors are of a palish yellow colour. In. Lines. Length from nose to the root of tail 6 6 of tail 2 10 from nose to ear 1 4 of tarsi (claws included) 1 1 of ear 0 7 Width of ear 0 7½ Habitat, Chile, (_September_.) This species is about one-third the size of the last, it differs moreover in being gray instead of brown, and in having the posterior margin of the ear emarginated; the tail is also rather shorter in proportion. The skull[41] is figured in Plate 33, fig. 1, _a_, and 1, _b_; and fig. 23, _a_, Plate 34. Its length is 1 inch, 4½ lines; width 9¼ lines; length of nasal bones 6 lines; distance between fore part of incisors and the front molar (upper jaw) 5 lines; longitudinal extent of the three molars of upper jaw 3 lines; length of auditory bullæ 5¾ lines; length of _ramus_ of lower jaw (see Plate 33, fig. 1, _c_,), without incisors, 11½ lines. Fig. 23, _c_, Plate 34, represents the inner side of a _ramus_ of the lower jaw: fig. 1, _d_, Plate 33, is the lower jaw seen from above: fig. 23, _b_, Plate 34, is the same seen from beneath. This view is given to show the position of the descending ramus of the lower jaw—that it springs from the outer side of the alveolar portion, as in a great portion of the South American Rodents, such as _Dasyprocta_, _Myopotamus_, _Echimys_, _Chinchilla_, and also in that genus found in the West Indian islands, _Capromys_. Fig. 1, _e_, Plate 33, represents the molar teeth of the upper jaw, and fig. 1, _f_, those of the lower. “This species is abundant on the dry hills, partly covered with bushes, near Valparaiso.”—D. FAMILY—CHINCHILLIDÆ. LAGOSTOMUS TRICHODACTYLUS.