ON TWO RECENTLY DESCRIBED MICE FROM ST. KILDA 133 



binomial or for a trinomial treatment I am less certain, until I have 

 had time to study the musculus- or sylvaticus-^KA Mice of the whole 

 Palasarctic Region. For many reasons it would seem convenient to 

 apply the trinomial system to all forms which can be clearly shown 

 to be local developments of any other form. By such a method a 

 clue is given to the relationships of the various local forms a 

 matter of no small importance to the student of a large and difficult 

 genus like that of Mus. On the other hand, we have, in the present 

 instance, two forms which, although obviously coming within the 

 above definition, are perfectly isolated and do not intergrade with 

 the parent form. Regarded from this point of view, they have as 

 much claim to be accorded full specific rank as any other island 

 species, and the latter is, perhaps, the most satisfactory method 

 whereby to deal with them. 



The following are the names which I propose : 



Mus HIRTENSIS, Barrett-Hamilton. 



Closely allied to Mus hebridensis, from which, however, it differs 

 in its slightly larger size, as stated above, and also in the greater 

 amount of buff or yellowish -brown coloration on the under side. 

 Like Mus hebridensis, it differs from typical sylvaticus in the more 

 uniform coloration of the upper surface of the body, in the absence 

 of the clearly defined white under side, and in the longer feet and 

 smaller ears. The skull is similar to that of Mus hebridensis, but 

 appears to be larger, equalling in size that of the largest specimen of 

 Mu s flavicollis. 



Mus MURALIS, Barrett-Hamilton. 



In shape and proportions allied to Mus musculus, but more 

 robust and larger in size. In general colour of the upper surface 

 resembles a dark specimen of Mus sylvaticus typicus, the base of 

 the hairs being of the same colour as in that species, but having 

 the extremities of the majority of a sepia-brown colour ; mixed 

 among these are a certain proportion of rufous-tipped hairs, which 

 give the animal a grizzled appearance. The colour of the under 

 surface is very remarkable, being buff, clearly separated by a well- 

 marked line of demarcation from the colour of the upper surface of 

 the body. The skull, as compared with that of typical Mus 

 musculus, is remarkable for the greatly exaggerated narrowness of the 

 posterior opening of the nostrils. 



The interest possessed by these two Mice, which undoubtedly 

 represent local developments of Mus sylvaticus and Mus musculus, 

 will be better appreciated if I briefly discuss the variations to which 

 Mus musculus and sylvaticus are subject in other localities. 



I assume, however, from the outset that in neither case am 1 

 dealing with an animal which may have been recently introduced 



