336 Mr. O. Thomas on the South-American white to their bases. Hands and feet grey. Tail blackish brown above, whitisli below. Skull very like that of A. varius ; interorbital region not quite so broad, similarly square-edged, without definite ridges. Incisors slightly more slender. Dimensions of the type : — Head and body 98 mm, ; tail 79 ; hind foot 24 ; ear 18. Skull : greatest length 28'5 ; condylo-incisive length 27 ; zygomatic breadth 14-.'7 ; nasals 10"2 ; interorbital breadth 4*6 ; breadth of brain-case 12'2 ; palatilar length 12*6 ; palatal foramina 7 ; post-foramiiial palate 4 ; upper molar series 4'9. Nab. Tucuman. Type from Villa Nouges, San Pablo. Alt. 1200 m. I'l/pe. Adult male. B.M. no. 2. 1. 5. 8. Original num-ber 1365. Ctdl.cted 22nd September, 1901, by L. Dinelli. Presented by Oldtield Thomas. Six specimens. While this species resembles A. varins in the buffy or clay-colour of the dorsal suffusion, it differs by the contrasted grey of the head, shoulders, Hanks, and hips, which give it a decep-tive resemblance to the species of Abrothrix. But there is iro doubt that its nearest relationship is with the first-named animal. Its peculiar white cliin is unique, but T should not lay very much stress on this character, which may easily prove variable. XL. — The Groiqnnff of the South-American Muridse commonly reftrred to Akodon. By Oldfield Thomas. (Published by permission of the Trustees of the British Museum.) In his 'Mammals of Patagonia' Dr. J. A. Allen has drawn attention to the composite nature of what is usually called Akodon, and has indicated one or two groups which he thinks ought to be generically or subgenericaliy distinguished — for example, those named below Chalcomys and Chroeomys. I have now had an opportunity to examine the whole group, which I find — uniform as it is in essential characters, and notably in the hypsodontism and general structure of the teeth — may be subdivided into seven genera. Of these, three — Akodon, Abrothrix, and Zygodontomys — were previously provided with names, while four need new ones. Two of these four are inhabitants of the Andean plateau and two o£ the lowlands of Brazil. A special subgenus is also made for A. cerosus, A. urichi, and their allies.