472 MURIDA—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 mamme confined to the 
inguinal region, the four pectoral mamme of J/icrotus being 
absent. In the least modified species the skull differs little from 
that of many of the more primitive species of AZzcrotus; 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; ' and mm’ are 
normal, the latter sometimes with a vestigial third inner angle 
recalling the agrestzs group; m7 
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. mz, has three 
instead of five closed triangles, 
the pair behind the anterior loop 
being broadly (in European) or 
half (in some American species) 
Pie an — Dom whore. SIGH conffuent (Fig. 77); ms has only 
TEETH ; crown view; Io times life size. TWO closed triangles, the anterior 
(Drawn by M- A. C. Hinton.) pair being half-confluent ; 7, is 
normal. This description applies to all the known American 
and European species, but in Asia species occur which 
partially bridge the gap between Pztymys and Mocrotus. 
Thus Thomas has. found six mamme (including a pectoral pair) 
in his P. mazyorz (described from Trebizond, Asia Minor, Anu. 
and Mag. Nat. Hist., April 1906, 419), and eight (four pectoral) 
in his P. carrutherst (from the Hissar Mountains to the east of 
Samarkand, Turkestan (Aun. 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 J/zcrotus, 
viz., WZ. irene, millicens, and onzscus, have been described by 
Thomas from eastern Asia; in these the dental characters of 

