3G4 Messrs. Barrett-Hamilton and Ilinton on new 



by a number of specifically or subspecifically distinct forms. 

 Among such relicts E. alstoni is one of the least modified, 

 and appears to have its closest ally in E. norvegicus, the 

 Skandinavian representative of the ancient type. The 

 Ilaasay Bank-mouse carries the story a stage further. A 

 careful analysis of the dental and cranial characters of all the 

 available species of Evotomys recently made by one of us 

 leaves no room for doubting that E. alstoni and E. erica are 

 more nearly allied to each other than is either of them to any 

 other known form. Of them, the former is the more primitive; 

 the latter appears to have adapted itself to subsistence upon a 

 coarser diet than that affected by mof-t species of Evotomys. 

 For this purpose the cheek-teeth have become larger and 

 stronger ; the muscles of the jaws have therefore acquired 

 greater strength, and this in turn has, by demanding in- 

 creased size and strength from those parts of the skull to 

 which the muscles in question are attached, moulded the 

 skull in a peculiar way. Thus the growth of the anterior 

 part of the temporal muscle has caused more extensive con- 

 striction of the interorbital region, produced longer and more 

 prominent postorbital crests, and stimulated growth of the 

 upper border of the jugal. The increased development of 

 the masseter has caused deepening of the rostrum, widening 

 of the outer wall of the infraorbital canal, and expansion of 

 the zygomata, in addition to the change in the angular 

 process of the mandible described above. To the growth of 

 the pterygoid internus is due the widening of the pterygoid 

 fossae and the narrowing of the interpterygoid space; further, 

 because of this extension the eustachian tube, in order to 

 keep in connection with the pharynx, has grown further 

 forwards and inwards, and so has produced the change noted 

 in the form of the bulla?. 



Microtus agrestis mial, subsp. n. 



Hah. ElGG, Inner Hebrides. " Not entirely nocturnal ; 



common in big heather" {Montague) . 



Material examined. Ten collected by Mr. P. I). Montague. 



Dimensions : — 



Head and Hind 



body. Tail. foot. Ear. 



No. 63, male, 30th March, 1913 108 32 13 13 



No. 66, „ 31st „ „ 116 32 18-5 13 



No. 75, „ 7th April, „ 108 34 18 13 



No. 49, female, 29th iM arch „ 104 31 195 13 



No. 50, „ 28th „ „ 110 32 19 12 



No. 51, „ 29th „ „ 106 32 19 12-5 



No. 62, „ 30th „ „ 103 30 19 13 



No. 64 (type), female, 30th March, 1913. 113 36 20 12 



No. 65, „ „ „ „ 110 31 20 13 



No. 67, „ 31st „ „ 100 29 18 11-5 



