This rat is very closely allied to _Mus decumanus_, and I think may possibly prove an extraordinary local variety of that animal. Having but one skin, and no skull, I am unable to satisfy myself on this point. Its size, as will be seen by the admeasurements, exceeds that of the common rat, or, rather, it exceeds ordinary specimens of that animal, for I have seen _some_ which were equal to it. “It was killed near Maldonado, where it frequented holes in the sand hillocks near the shore. It is likewise found on the island of Guritti. If ships are ever infested with these monstrous rats, the above-mentioned localities are very likely places to have received colonies by such means. An old male weighed fifteen ounces and three quarters. The ears of this rat, when alive, were of a pale colour, which made a singular contrast with the black fur of its body.”—D. 3. MUS JACOBIÆ. Mus decumanoïdes,[15] _Waterh._ in “Catalogue of the Mammalia preserved in the Museum of the Zoological Society of London.” _M. suprà fuscus, griseo-lavatus, subtùs albus: pedum pilis sordidè albis; caudâ corpore cum capite paulò longiore; auribus mediocribus: pilis perlongis in dorso crebrè inter cæteros commixtis._ DESCRIPTION.—The general tint of the upper parts of this rat, is grayish brown, (very nearly resembling that of _Mus decumanus_); the longest hairs, which on the hinder portion of the back are one inch and a half in length, are black; the ordinary hairs are black at the apex, there is then, on each hair, a considerable space occupied by pale yellow, and the remaining, or basal portion, is grayish white; the under fur is gray: the hairs of the chin, throat, and under parts of the body, are white, and without any gray colour at the roots: the feet are covered with dirty grayish hairs: the tail, which is slender, is very sparingly furnished with minute black hairs, both above and beneath: the ears are of moderate size, of a brownish flesh-colour, and, to the naked eye, appear to be destitute of hair. The hairs of the moustaches are most of them black at the base, and grayish at the apex. In. Lines. Length from nose to root of tail 7 6 of tail 7 6 of tarsus 1 4¼ of ear 0 7½ from nose to ear 1 7½ Habitat, James Island, Galapagos Archipelago, Pacific Ocean, (_October_.)