472 



MURID^— LOCALLY EXTINCT VOLES 



tion for burrowing. The usually short fur is denser and finer, 

 the ears and tail shorter, the eyes smaller, and there are usually 

 only five plantar tubercles, and four mammae confined to the 

 inguinal region, the four pectoral mammae of Microtus being 

 absent. In the least modified species the skull differs little from 

 that of many of the more primitive species of Microtus ; the 

 inter-orbital region is relatively broad, the temporal ridges little 

 developed, the brain-case large, often depressed, and smooth. 

 In the more specialised forms the fossorial characters of the 

 skull (oblique truncation of occiput, straightening of the upper 

 incisors, and the characters correlated with these two) are 

 marked. The cheek-teeth grow persistently ; ot^ and n^ are 

 normal, the latter sometimes with a vestigial third inner angle 



recalling the agrestis group ; ni 

 has three outer and three or 

 four inner salient angles, and 

 when most reduced its first outer 

 triangle tends to atrophy, and 

 is more or less confluent with 

 the inner triangle, m-^ has three 

 instead of five closed triangles, 

 the pair behind the anterior loop 

 being broadly (in European) or 

 half (in some American species) 



Fig. 77. — Pitymys suhterraneus. RIGHT a ^ it:' __\ 1 i 



Upper and Left Lower Cheek- confluent (Fig. ^-j) ; m, has only 



Teeth ; crown view ; 10 times life size, two closed triangles, the anterior 



(Drawn by M. A. C. Hinton.) p^j^. ^^j^^^ half-COnfluent ; M, is 



normal. This description applies to all the known American 

 and European species, but in Asia species occur which 

 partially bridge the gap between Pitymys and Microtus. 

 Thus Thomas has found six mammee (including a pectoral pair) 

 in his P. majori (described from Trebizond, Asia Minor, Ann. 

 and Mag. Nat. Hist., April 1906, 419), and eight (four pectoral) 

 in his P. carrutkersi (from the Hissar Mountains to the east of 

 Samarkand, Turkestan [Ann. and Mag. Nat. Hist., March 

 1909, 263); in both these forms also the tail and ears are 

 rather long. Three species, ranged at present with Microtus, 

 viz., M. irene, inillicens, and oniscus, have been described by 

 Thomas from eastern Asia ; in these the dental characters of 



