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Show 692 MR. O. THOMAS ON A NEW [Dec. 14, in which they reach to the end of the first phalanx of the fourth digit \ Tail about equal in length to the body without the head. Quality and coloration of the fur almost exactly as in pale specimens of Sigmodon hispidus, Say and Ord, to which this species bears a most remarkable resemblance. Fur of the back of two kinds : the woolly underfur dark slaty blue ; and of the longer hairs some are black for their whole length, and others have a long yellowish-white tip. Belly white, with a slaty tinge at the roots of the hairs ; sides of the muzzle yellowish ; whiskers brown. The anterior edge of the root of the zygoma is rather deeply emarginate in front, though not so much so as in R. cuniculoides. The palate agrees more with that of R. chinchilloides, ending just behind the molars, and being less ridged and excavated than in R. cuniculoides. Posterior nares not narrowed by the approximation of the pterygoids. Incisive foramina short, ending in front of the first molar. Pterygoid fossae deep. Frontal bones with a strong concavity just behind the orbital constriction, as shown in the drawing (see fig. 2). Fig. 2. Skull of Beithrodon alstoni. The teeth are very different from those of any of the other species of the genus, being much shorter and broader, so that the whole length of the molar series is contained twice in the distance between the incisors and the first upper molar, while in none of the others is it contained more than once and a half; and the second upper molar is distinctly broader than long, while in the other species the opposite obtains. The molars of the type specimen are too much worn down for their pattern to be described ; but the woodcut (fig. 2) shows their general shape. The upper incisors have a deep and distinct groove down the centre of their front surface, a groove much more strongly marked than in any of the other species. 1 This character, which Waterhouse appears to have overlooked seems to carry out stdl further the parallelism between Sigmodon and Oryzomys'on the one hand and the two " subgenera " of Beithrodon on the other, which Dr Coues drew attention to when subdividing this genus (see infra p 694) |