OCR Text |
Show 694 MR. O. THOMAS ON A NEW [Dec. 14, one large genus, with the present genera Holochilus, Hesperomys, Siymodon, Reithrodon, and Ochetodon each representing a subgenus of greater or less extent. Of these names Sigmodon would have to be the one to stand for the genus, antedating Waterhouse's appropriate name Hesperomys by 14 years. As, however, we must for the present consider R. alstoni as a true Reithrodon, the next point we have to consider is its relationship to the other described species. The only species of this rare genus hitherto known are Waterhouse's three original ones, the North- American mice described as belonging to it by Baird and De Saus-sure having been separated by Dr. Coues as a distinct genus under the name of Ochetodon l. At the same time that he founded this last genus, Dr. Coues also divided the true Reithrodon into two subgenera-Reithrodon proper, containing R. typicus and R. cuniculoides, and Euneomys, containing R. chinchilloides only. This subdivision certainly seemed to be justified by Waterhouse's figures and descriptions ; but now that R. alstoni has to be arranged with the others it is at once plain that these subgenera cannot stand as such, that species being, as far as regards the subgeneric characters used by Dr. Coues, precisely intermediate between them. Thus, it has the concave front edge of the zygoma-root, the deep pterygoid fossae, the rounded descending process of the lower jaw, and the short fifth toe of Reithrodon as restricted, and the short incisive foramina, the short palate, and the uncontracted posterior nares of Euneomys. Unless, therefore, we are prepared to make a third subgenus for the reception of this species, a course which I think is hardly necessary, we must abolish the subgenera above named and include all the species under Reithrodon proper. With regard to the specific distinctness of R. cuniculoides and R. typicus, about which Dr. Coues rather naturally expresses some Front part of skull of B. typicus. doubt, it unfortunately happens that the type of the latter in the British Museum is in an extremely bad condition, having all the cranial and palatal portions of the skull broken away ; but there remains enough to show that the nasal region, though similar, is not identical with that of R. cuniculoides, and that the muzzle is some- 1 Proc. Ac. Nat. Sci. Phil. 1874, p. 184. |