In. Lines. Total length 1 2 Width 0 8½ Length of nasal bones 0 6⅓ of incisive foramina 0 4 Distance between the outer surface of the incisors and the first molar tooth, upper jaw 0 4½ Longitudinal extent of the three molars of the upper jaw, taken together 0 2¾ Length of a ramus of the lower jaw without the incisor 0 8 _General Observations upon the foregoing Species of Muridæ._ 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.