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Show 1893.] MR. O. THOMAS ON MAMMALS FROM CENTRAL PERU. 339 genus or subgenus Habrothrix appears to me quite unaccountable, especially as he is one of the authors who consider that tbe majority of tbe different groups of what used to be called " Hesperomys " should rank as distinct genera. A short diagnosis of Habrothrix, based on the typical species, H.longipilis, is given in P. Z. S. 1884, p. 450, and it will be seen that Ichthyomys stolzmanni and /. hydrobates agree with that diagnosis in scarcely a single character of importance, even though it was not drawn up with the idea of any antithesis to such a specialized swimming form as the present. The very noticeable resemblance in the structure of the anterior zygoma-root which Ichthyomys bears to Hyclromys is a point well worthy of remark, for while this resemblance practically amounts to identity, yet there can be no suspicion that the two have a common origin, or can be other than a very remarkable case of parallelism. This case is the more remarkable as the structure of this region has been used by all the best authorities as a character of primary importance in dividing the Myomorph rodents into smaller groups, so that it will not be readily looked upon as one of little stability. Nevertheless, in the present instance we have two Murines, alike in their mode of life but derived from quite different ancestors, developing independently exactly similar infraorbital foramina'. The presumed ancestor of each of the two highly specialized forms under consideration, Xeromys of Hydromys, and Habrothrix (or some ally) of Ichthyomys, both have typically murine infraorbital foramina. ICHTHYOMYS STOLZMANNI, sp. n. (Plate XXVIII.) Size and general proportions much as in the common Black Eat (Mus rattus). Whiskers long, strong, and numerous, silvery white with the exception of a few of the upper ones, which are brown. Ears very small and narrow, when laid forward they do not reach halfway towards the eye. Hands with the fingers quite free and unwebbed ; pollex with a sort of elongated nail, not long enough to be called a claw; other digits with sharp curved claw-s ; third and fourth fingers subequal, second reaching to the middle of the second phalanx of the third, fifth to the base of the same phalanx of the fourth ; palm naked, with the usual five pads. Hind feet broad, fan-shaped ; unwebbed part of toes broadly ciliated marginally ; soles naked, with five large low rounded pads, the usual smaU postero-external pad absent. Tail about as long as the head and body, thick, cylindrical, its terminal half below with stiff elongate bristles. Palate-ridges 3-3. Mammae doubtful, owing to part of the abdominal wall having been cut away, but there is one pair just behind the axillae, and another near the vulva; 1 Dr. Winge (t. c. p. 22) ingeniously suggests, as the reason of the enlargement in the lower part of the foramen, the increased size of the nerve which supplies the prominent whisker-bristles. Although no doubt true so far as it goes, this explanation does not appear to m e quite to cover the case, especially as Fiber, similar in habits and with nearly equal whisker development, has a highly typical Murine foramen. |