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Show 422 MR. G. E. H. BARRETT-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 tbe 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 31. agrarius Pallas. The under molars come very close to those of 31. sglvaticus, 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 that 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 Muridce, being even more specialized than the modern species. TABLE OF DIMENSIONS. The dimensions of the specimens examined during the preparation 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 sglvaticus 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. |