OCR Text |
Show 536 MR. O. THOMAS ON THE [May 3, I have, with some doubt, placed M. infralineatus under var. rufescens, as the colour characters given are quite unimportant, and the proportions might be those of an overstretched half-grown individual; it is, however, very possible that I may be wrong, and that it is a distinct species. Sir Walter Elliot informs me that this species seems to be gradually giving way before the more powerful M. decumanus, which has been introduced into the various Indian ports from the shipping, just as in Europe the same ubiquitous species has nearly exterminated the corresponding northern form Mus rattus. With regard to the specific distinctness of Mus alexandrinus from M. rattus, about which so much has been written, it seems to me that we have here a somewhat parallel case to that of M. urBanus and M. musculus. M. alexandrinus would seem to be a more tropical form of M. rattus ; but as it seems always to have much shorter, coarser, and more rufous hair, as compared with the black and shining fur of M. rattus, I have provisionally kept them distinct, though I have but little doubt that they will have finally to be considered as but one species. This, however, is a question not specially Indian ; and so at present I prefer to avoid expressing any definite opinion on the subject. The extraordinary variation of the nasal bones of this species has been mentioned above. Some authors have used the length of these bones as a specific character ; and certainly the extremes in this species are so great as to appear to be of importance ; but when a series is examined it will be found that no stress can be laid on this character. In order to prove how great the variation is, I have prepared the following Table; all the specimens measured were collected at the same time and place, and are obviously all of the same species. Measurements of Skulls and Nasal Bones of Darjiling Rats (Mus alexandrinus, var. nitidus) :- Percentage of nasal bones to rest of skull. 49 46 62 57 64 54 66 52 69 62 59 69 Total length of skull. millim. 31*6 33*5 39*0 40*0 41*9 42*0 42*5 42*6 44*0 44*5 44*7 48*0 Nasal bones. millim. 10*5 11*0 15*0 14*5 16*4 14*8 17*0 146 18*0 17*0 16*7 19*5 If we now arrange these percentages in order, we obtain the following series, in which it is perfectly impossible to draw any dividing line |