The molars of the lower jaw of _Mus bimaculatus_ agree with those of _M. Rattus_ as to the number of tubercles which they possess; they are, however, proportionately longer and narrower, and, when a little worn, these teeth, as well as those of the upper jaw, differ considerably from those of _M. Rattus_. In the last named animal, when the molars are slightly worn, the ridges of enamel run completely across the tooth, as in Figs. 18 and 19, Plate 34. Such is not the case in _M. bimaculatus_ at any age. As soon as the molar teeth are worn, the folds of enamel penetrate the body of the tooth on each side, and those of one side alternate with those of the other,—in fact, they very nearly resemble those of the _Hamsters_ (_Cricetus_). I have selected the molar teeth of _Mus Rattus_ and _M. bimaculatus_ for comparison, since I happened to possess specimens displaying both the young and adult states of each. But had I selected, on the one hand, almost any of the species brought from South America by Mr. Darwin, and, on the other hand, the _Mus musculus_ or _M. decumanus_, I should have had to point out the same distinctions—the former agreeing in dentition with _M. bimaculatus_, and the latter with _M. Rattus_. The differences pointed out, between the molar teeth of _Mus Rattus_ and those of _M. bimaculatus_, I cannot but consider as important, since all the Old World species of _Mus_ which I have yet had an opportunity of examining (and they are numerous) agree essentially with the former, whilst the only _Mus_ from S. America (excepting _M. Musculus_ and _M. decumanus_, which are carried in ships to all parts of the world) in which I have as yet found molar teeth like those of _M. Rattus_, is the _Mus Maurus_, and this it has been stated is possibly a variety of _M. decumanus_. Although as yet I have not met with species in the Old World possessing the characters of the South American _Muridæ_, among those of North America, several have come under my observation. The _Mus leucopus_, _Symidon hispidum_, and the species of _Neotoma_ certainly belong to the same group,[28] as does also the species of the Galapagos Islands, described in this work under the name _Galapagoensis_. These considerations have induced me to separate the South American mice from those of the Old World,—or rather from that group of which _M. decumanus_ may be regarded as the type,—and to place them, together with such North American species as agree with them in dentition, in a new genus bearing the name _Hesperomys_.[29] Whether this group be confined to the Western hemisphere or not, I will not venture to say, but I think I may safely affirm that that portion of the globe is their chief metropolis.