74 ZOOLOGY OF THE VOYAGE OF THE BEAGLE. 



General Observations upon the foregoing Species of Murida 



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 1 have made outlines of the molars, which will convey 

 a mor£ 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 

 Mns 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 



