When at Paris I examined what I believe to be the original _Mus Braziliensis_, since the specimen was labelled “_Rat de Brazil St. Hilaire, 1818_.” It agrees perfectly with the present animal excepting in being rather smaller, the length from the nose to the tail being 7 inches and 4 lines—the length of the tail is 7 inches 9 lines, and that of the tarsus is 1 inch 11 lines; this difference in the length of the body may arise from difference of age, or even of sex. In the Paris Museum I saw what appeared to me to be a variety of the same species in which the under parts of the body are white. I have been minute in my description of the _Mus Braziliensis_, since it is confounded by Desmarest, Fischer and Lesson with the _Rat troisieme_ or _Rat Angouya_ of Azara, which I believe to be a very different animal. The description given by the authors just mentioned are taken from Azara, who gives the following characters to distinguish the _Rat Angouya_: “Du museau à la queue, et sur les côtés du corps tout est brun-cannelle, parceque les poils ont une petite pointe cannelle; puis, ils sont obscurs et enfin blanc vers las peau. Toute la partie inférieure de l’animal est blanchâtre, plus claire sous la tête, et plus foncée entre les jambes de devant; le pelage est doux, très-serr, et le poil, qui est à la racine de l’oreille, cache le conduit de celle-ci.” In. Lines. Length from nose to root of tail (English measure) 6 0 of tail 6 6½ of ears 0 9¾ of tarsus (the claws included) 1 3¼ It appears from this description that the _Mus Angouya_ is a smaller animal, and differs both in colouring and proportions from the _Mus Braziliensis_. Brandt has figured and described a rat under the name of _Mus Angouya_, which in many respects agrees better with Azara’s description; there are, however, discrepancies in the dimensions. 22. MUS MICROPUS. PLATE XX. Mus micropus, _Waterh._, Proceedings of the Zoological Society of London for February 1837, p. 17. _M. suprà fuscus; subtùs cinerescenti-albus, pallidè flavo tinctus; pedibus pilis sordidè albis tectis, antipedibus parvulis; auribus parvulis; caudâ, quoad longitudinem, corpus ferè æquante, suprâ fuscâ, subtùs sordidè albâ._