In the foregoing descriptions I have endeavoured to convey an idea of the characters of the species of mice submitted to me for examination and description, by Mr. Darwin: there are, however, some points upon which I have been silent in my descriptions. I allude to the characters observable in the dentition. I have omitted to notice the various modifications in the structure of the molar teeth, because I found it would lengthen the descriptions to no good purpose, inasmuch as of almost all the species I have made outlines of the molars, which will convey a more clear idea than any verbal description can do.
Upon an inspection of the Plates, it will be seen, that by far the greater portion of the teeth figured, may be referred to one particular type of form or pattern, and that this pattern does not agree with that observed in the molars of Mus Rattus, M. decumanus, or M. musculus, whilst these three species agree essentially with each other.
In the young Black Rat (Mus Rattus), before the teeth are worn, the two anterior molar teeth, on either side of the upper jaw, present three longitudinal rows of tubercles, a central series of larger tubercles, and on each side of these, a row of smaller ones. The front molar has three of the larger tubercles arranged along the middle of the tooth; three smaller ones on the outer side, and two, on the inner side. The second molars have two central tubercles, two outer, and two inner ones. The posterior molar is nearly round, the body of the tooth consists of three principal tubercles, and one small tubercle, situated on the inner and anterior portion of the tooth.
The corresponding teeth in the young of Mus bimaculatus present a very different appearance; the molars, instead of having three longitudinal rows of tubercles, have only two. An idea of the appearance of these teeth may be formed by removing the inner row of tubercles from the molars of Mus rattus. We should then have, as in Mus bimaculatus, molars of a narrower form, the first tooth presenting six tubercles, the second, four; and the posterior tooth devoid of the small inner lobe; the opposing tubercles of each tooth, however, in M. bimaculatus, are of equal size.
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.