ARVICOLA 409 



plain ; for tlie teeth in de Filippi's specimen are of tlie ordinary 

 angular type. No other character, or at least none that cannot 

 be readily explained by differences of age in the individuals 

 examined, distinguishing the highland from the lowland form is 

 visible in the material available. The difference in the character 

 of the salient angles in the two sets of specimens is certainly 

 striking, but I think it is susceptible of another explanation than 

 that given by Thomas. In prismatic teeth the precise shape 

 of the angles and infolds which form the pattern visible at the 

 grinding surface depends among other things upon the angle at 

 which the plane of wear cuts the prisms. It is possible to get 

 from one and the same tooth very different patterns by slightly 

 altering the plane of wear. The cheek-teeth of Arvicola are ever- 

 growing, and they grow in two spirals, one more or less longitudinal, 

 the other transverse. If the. animals feed on hard substances or 

 dwell upon a sandy soil their teeth are worn down rapidly ; if the 

 food is soft, the wear is slower. In the one case growth at the base 

 may or may not be able to keep pace with the wear on the crown 

 of the tooth; in the other, growth may outstrip the wear; 

 in the two cases supposed the angle of the plane of wear will 

 necessarily be different and the pattern will differ accordingly. 

 A slight difference in station may necessitate a great difference in 

 food and may therefore give rise to a considerable difference in 

 both the exact shape of the salient angles and infolds and in 

 the apparent size of the teeth. 



7h. Arvicola terrestris scythicus Thomas. 



1914. Arvicola terrestris scythicus Thomas, Ann. Mag. N.H., [8], 13, 

 p. 568. 



Type.~BM., No. 14.5.10.154; adult female, collected May 

 5, 1913, by W. RUckbeil, and presented by the Hon. N. C. Roths- 

 child. 



Type locality. — Djarkent, Semiretchensk, Central Asia. 



Range. — Besides the typical series from Djarkent there are a 

 number of specimens in the Museum, collected by G. A. Burney 

 at Irkutsk, and at Alzamai on the banks of the Jenissei River, 

 which appear to be referable to this form. No doubt A. t. 

 scythicus has a wide range in Central Asia. 



Characters. — Size large, hind-foot 34 mm., condylo-basal 

 length 42 mm. General colour about as in A. a. amphibius or 

 as in pallid examples of A. t. terrestris; the reddening of the 

 cheeks characteristic of A. t. terrestris well marked. Tail 

 black, scarcely lighter below, its tip usually with a small white 

 pencil. 



Skidl equalling that of A. amphihius in size, but with the 

 fossorial characters of A. t. terrestris intensified, the incisors 

 more protruding and the occiput more oblique than in the typical 

 subspecies. 



V.L. E E 



