This species is scarcely equal in size to a full grown common black rat, (_Mus Rattus_), the head is rather shorter in proportion, the tarsi are smaller, and the tail is longer. In the character of the fur, and length of the hairs, it _very_ closely resembles that species: the ears are larger than in _M. decumanus_, and about equal to those of _M. Rattus_. In having the hairs of the under parts of the body of an uniform colour, (i. e. not gray at the base,) it resembles the _Mus Tectorum_ of Savi; but the large size of that animal, the greater length of the fur, and its colouring, all serve to distinguish it from the present species, which I may here observe, is truly an old world form, and very distinct from another species, also from the Galapagos, which is hereafter described. “It is very common in James Island, but is not found on all the islands, if on any other in the Archipelago. Although its appearance is so like that of the common rat, yet its habits appear to be rather different: it is less carnivorous, and does not appear to be so strongly attached to the habitations of man. This island was frequented, about one hundred and fifty years since, by the vessels belonging to the Bucaniers; so that the common rat might easily have been transported here. And if a very peculiar climate, a volcanic soil, and strange food, can together produce a race, or strongly marked variety, there is every probability of such change having taken place in this case.”—D. 4. MUS (RATTUS _var._?) INSULARIS. _M. suprà grisescenti, colore subtùs dilutiore; tarsis purpureo-nigris: caudâ corpus cum capite æquante: auribus mediocribus: vellere molli._ DESCRIPTION.—No. 1. The general colour of this animal is what might be termed black, there is, however, an obscure purple-brown hue on the upper parts of the body, and the sides and under parts have a grayish tint, the hairs covering the feet above are of an uniform deep purple-brown, almost black. All the hairs of the body are gray at the base: the hairs of the moustaches are long and numerous, and of a black colour, having one or two white hairs intermixed: the ears are of moderate size, and very sparingly furnished with minute dark hairs: the tail is long and slender, and has small, scattered, bristly hairs, of a brown-black colour. In. Lines. Length from nose to root of tail 7 0 of tail 6 6 of tarsus 1 3½ of ear 0 7 from nose to ear 1 6