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Show 1888.] MR. O. THOMAS ON A NEW GENUS OF MURID/E. 133 Madagascar Muridae wholly belong. Thus Cricetine Muridae, now at their highest development in America, were certainly paramount at one time in Africa, where the intermediate Deomys nowr occurs, as in other parts of the Old World, and have only recently (since the separation of Madagascar) been supplanted by the more highly specialized Mures. This supplanting, however, has never been quite completed, since in the Old World there still survive several Cricetine genera, mostly much modified (e. g. Arvicola and its allies), but in some cases with scarcely any appreciable changes at all (e. g. Mystromys, Cricetus, and, so far as its teeth are concerned, Lophiomys). And this brings me to the second part of the subject, a part that for reasons of nomenclature is much to be regretted, but which obviously has to be investigated, namely, as to the amount of generic distinction actually existing between the Old-World Cricetus and the New-World Hesperomys. So far as I know, they have hardly ever been properly compared, being always presumed to be distinct in the absence of proof to the contrary. On trying to tabulate the differences, however, and keeping in mind at the same time the extent of variation found in the American species, I find that one by one they vanish into thin air, leaving no distinctive character whatever. The large cheek-pouches of Cricetus are commonly looked upon as a marked characteristic of the genus, but, although small, they occur, distinct and well developed, in many species of Hesperomys. especially in those from the extreme north l. The teeth of Cricetus, again, are very like those of many of the species of Hesperomys, especially if those of a " Cricetulus " (e. g. C. phceus) be compared with those of some of the species of " Oryzomys"2, baving similarly six paired cusps, while those of " Vesperimus" 3 have only five, the cusp A 2 being here obsolete or united with A 1. A still closer resemblance, amounting in fact almost to identity, exists between the structure of the teeth in Cricetus and in the Dormouse-like subgenus Rhipidomys1. In the skull the shape of the infraorbital toramen, of the supraorbital ridges, and of the palate may be equally easily matched among the numerous and widely varying species of "Hesperomys." Finally the peculiar shortness of tail characteristic of Cricetus is almost, if not quite, equalled in the North-American subgenus Onychomys0. The inevitable conclusion is thus forced upon us that the genus Hesperomys must be abolished altogether, and the species united 1 In the widely spread white-footed Field-Mouse of N. America {Cricetus leu-copus, as it will now have to be called), northern specimens have deep and distinct cheek-pouches, while in southern ones they are, at least in spirit-specimens, almost inappreciable. See Allen, Bull. Mus. Comp. Zool. i. p. 229 (1869), and Coues, Mon. N. A m . Rod. p. 67 (1877). 2 E. g. Cricetus palustris or longicaudatus. 3 Among which are included Cricetus leucopus, californicus, aureolas, taylori, michiyanensis, truei, aztecus, and mexicanus. 4 See, for example, the teeth of Cricetus {Bhipiidomys) leucodactylus, latimanus, sumickrasti, mastacalis, or sclateri. 3 Of which the species are Cricetus leucogaster and C. torridus. |
