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Show 464 DR. E. A. GOELDI ON T H E [June 5, (Hemiurus, Microdelphys) of most authors, and may say that this species is here almost more frequent than Micoureus pusillus. Mr. 0. Thomas (I. c. p. 363 et seq.) admits two species of three-lined Peramys-a larger (P. americanus) aud a smaller (P. iheringii), giving a good figure of the latter on plate iv. The habitat of P. americanus is mentioned as " Brazil " (type not in existence), and that of P. iheringii as " South-Brazil," especially Eio Grande do Sul (type in British Museum). Mr. Thomas says of P. iheringii:-" This species appears externally to be merely a dwarf form of P. americanus, but it may always be distinguished not only by its much smaller size, but by the different shape of the skull, and especially by the marked flattening of the frontal region." I confess m y scepticism about the opinion of Mr. Thomas, and I think I have serious reasons to do so. Concerning the measurements this author states that he follows Burmeister, " Erlaiit." p. 84, giving the dimensions of the same individual. But Burmeister writes :-whole length 7" (that is 7 Prussian inches), tail 2" 2'" (2 Prussian inches 2 lines). The Prussian inch is 2-6154 cm., and the dimensions are equal therefore to 183 m m . (whole length) and 57'53 m m . (tail), while Mr. Thomas reduces them to 189 m m . and 59 m m . - a perceptible difference for a small animal. The dimensions of a second individual preserved in the Berlin Museum are said to be 168 m m . (whole length) and 61 m m. (tail). On the other hand, the dimensions of the type specimen of P. iheringii (adult?) described by Mr. Thomas are 110 m m. (whole length) and 43 m m . (tail). So long as the minimum size of P. americanus and the maximum size of P. iheringii are not exactly determined by large series of authentic adult specimens, a definite conclusion seems to be a very difficult matter; and as the question now stands it may be allowed to interpret the difference of size as the expression of different age, the more as the shape of the skull is evidently also affected by growth, as proved by B. Hensel in his memorable investigations. I see that H . Winge shares this opinion, writing:-" P. iheringii, Thomas, synes at stemmeganske med smaa Individer af Hemiurus tristriatus" ('Pungdyr,' p. 108) ; and confronting the figures of skulls given by Mr. O. Thomas (Catalogue, pi. xxvii. fig. 8), H . Winge (' Pungdyr,' pi. ii. fig. 9), and Burmeister ('Erlauteruugen,' pi. xi. fig. 7), I am unable to find any other essential character to warrant the admission of two different species. I have had occasion to examine dozens of three-striped Opossums of all ages, and I can assert the existence of variations in colour and size. As regards the colour, young individuals are frequently seen with a yellowish or greyish tinge and exceedingly well-defined and conspicuous black stripes, just as in figure 2 of plate iv. of the Catalogue of Marsupials in the British Museum. Older individuals, generally more rufous, often show only pale stripes, and, not rarely, specimens will be met with apparently stripeless, the stripes only appearing against the light. I frankly confess that I |