1899.] THE MICE OF ST. KILDA. 81 



on the two Mice from St. Kikia I can have no doubt whatever ; 

 but as to whether they are fit subjects for a binomial or for a 

 trinomial treatment I am less certain, until I have had time to 

 study the muscidus- or sylvaticus-Yike Mice of the whole Palse- 

 arctic Eegiou. 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 aboA^e definition, are 

 perfectly isolated, and do not, so far as we kuow, intergrade with 

 the parent form. Eegarded 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, sp. nov. (Plate IX. fig. 1.) 



Closely allied to Mus hehridensis, 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 underside. 

 Like Mus hehridensis, 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 underside, and in the longer 

 feet and smaller tars. 



The skull is similar to that of Mus liehridensis, but appears to 

 be larger, equalling in size that of the largest specimen of Mus 

 JlavicoUis. 



The type is No. 94.7.16.1 (British Museum coll.), the young 

 male first collected by Mr. J. Steele Elliott. 



Mus MUEALis, sp. nov, (Plate IX. fig. 2.) 



In shape and proportions allied to Mus musmdics, but more 

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

 resembles a dark specimen of Mus sylvaticus typiais, 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 type is No. 534 of my own collection : it is an adult female 

 procured for me in 18S8 by Mr. Henry Evans. 



Proc. Zool. Soc— 1899, No. VI. 6 



