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 gra- 

 dually 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 tro- 

 pical form of ill. 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 con- 

 sidered 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 variationofthenasalbonesof 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 exa- 

 mined 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 Eats (Mus 

 alexandrinus, var. nitidus) : — 



Percentage of nasal bones 

 to rest of skull. 



49 

 46 

 62 

 57 

 64 

 54 

 66 

 52 

 69 

 62 

 59 

 69 



If we now arrange these percentages in order, we obtain the follow- 

 ing scries, in which it is perfectly imjiossible to draw any dividing line 



