THE NORTHERN GRASS MOUSE 431 
measured from the inter-orbital constriction to the condyles is not less 
than the zygomatic breadth. 
In the teeth the upper incisors project slightly ; the cheek-teeth are 
moderately large, with sharp salient angles and well-cut folds. In the 
maxillary series #1 is decidedly the longest, 7? and 7? being subequal 
in crown-length. 7! and 7? are composed of the same elements 
as in Evotomys, but possess, the former variably, the latter invariably, 
a more or less reduced, supplemental postero-internal prism, forming a 
conspicuous and very distinctive third inner salient angle in 7”. 
[European species with this pattern in mm? have sometimes been 
placed in a sub-genus named Agvicola or Sylvicola.] 
m* begins with the usual transverse loop and three closed triangles ; 
its posterior loop is long, and cleft internally by a deep fold, so that in 
the simplest form the tooth terminates 
in a strongly crescentic formation ; it 
possesses three infolds with four salient 
angles on each side, of which the pos- 
tero-external is weak and frequently 
obsolete. In a more complicated and 
rarer form the posterior inner fold 
extends across the loop to its outer 
side, thus isolating a second inner closed 
triangle followed by a simple posterior 
loop, which sometimes bears an incipient 
internal fold. 
In m, the large anterior loop is cleft py, 6s.— RIGHT CHEEK-TEETH OF 
bya deep internal and, more posteriorly, Microtus agrestis (A, upper; B, lower: 
by a shallower external re-entrant angle. 7 times life size.) 
This gives the tooth two anterior rounded projections, and behind them 
two shallow folds, in addition to the usual well-defined nine salient angles 
and seven infolds. Sometimes the shallow folds of the anterior loop 
may be deep enough to isolate a third outer triangle, so that the loop 
becomes a crescent resembling that of mz? and a rudimentary sixth inner 
fold appears ; rarely there may be a supplemental fold on the extreme 
antero-external border. (See Fig. 65.) 
m, is of the usual type, but as an abnormality its anterior external 
triangle may develop a slight inner basal projection so as to produce 
a small additional inner salient angle. In m, the salient angles and 
folds are bilaterally very unequal, being much deeper on the inner 
side, and with the antero-external angle usually obsolete. 
Geographical variation :—There are seven more or less differentiated 
sub-species. Two have been described above under Destrzbutcon. The 
other five are British. 
Status :—The arrival of a newer and more vigorous competitor in 

