422 ME. G. E. H. BAEBETT-HAMILTON ON [Apr. 3, 



young and unworn. The first upper molar is more hypsodont 

 than either of the next two. 



In the next oldest set of teeth the lateral grooving is still 

 evident, but is not nearly so deep as in the youngest ; while in 

 quite old and worn teeth the grooves are no longer visible. As 

 they are worn down, the teeth get smaller and at length become 

 separated from each other. The internal convexities of the first' 

 molar become less distinct, and the teeth-pattern of the crowns is 

 gradually obliterated. Altogether, the upper teeth remind me 

 very strongly of those of M. agrarius Pallas. 



The under molars come very close to those of M. sylvaticus, but, 

 like the upper, are more hypsodont in character. The tubercles 

 of the first tooth were probably arranged as in M. sylvaticus, and 

 the anterior unpaired tubercle is present. There is nothing to 

 call for comment in the last two under molars, the tubercles of 

 which, as in modern species, seem to have been 4 for molar 2, 

 and 3 for molar 3. 



The animal was larger than M. sylvaticus, and in its dentition 

 shows resemblances both to tha,t species and M. agrarius, so that it 

 may, perhaps, have been a direct offshoot from a common stock 

 from which both species have later sprung. In other respects, 

 however, it seems to throw no light on the ancestry of Palaearctic 

 Murida>, being even more specialized than the modern species. 



Table or Dimensions. 



The dimensions of the specimens examined during the prepara- 

 tion of this paper are included in a single table, so as to be more 

 convenient for purposes of comparison. By the careful averaging 

 of a large series of British examples, I have tried to give figures 

 which shall be useful as a standard with which may be compared 

 those of other subspecies. I am conscious that this standard 

 would have been more satisfactory had it been taken from 

 specimens from a single locality, instead of from those procured 

 all over Great Britain. It may, for instance, be some day found 

 that the average dimensions of specimens from North and South 

 Britain are different ; but I have bad to make the best use of my 

 material, and I think the figures given are a fair average for the 

 ordinary Mus sylvaticus of the British Islands generally. 



It is not easy to explain the large difference between the 

 minimum and maximum dimensions given in the case of M. s. 

 intermedins; but it should be borne in mind that as long as we 

 have to deal with a series of specimens measured by a numerous 

 band of collectors, so long will the individual factor tend to 

 increase the gap between minima and maxima. I venture to 

 think that the number of specimens which have been dealt with is 

 sufficient to make the averages trustworthy and useful for purposes 

 of comparison. It is to them that we must look for an idea as to 

 the true appearance of an ordinary individual of a particular 

 species dealt with, and they are not at all affected by the 

 occasional occurrence of giants or pygmies, the measurements of 

 which counterbalance one another. 



