74 
ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 
General Observations upon the foregoing Species of Muridce. 
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. bimi- 
culatus , 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, propor- 
tionately 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 
