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 m'- is decidedly the longest, m^ and m^ being subequal 

 in crown-length. m^ and m^ 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 w;^. 



[European species with this pattern in m^ have sometimes been 

 placed in a sub-genus named Agricola or Sylvicolai\ 



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 <^/T~ — }ni — 

 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 4&* — »23 

 loop, which sometimes bears an incipient 

 internal fold. 



Inm^ the large anterior loop is cleft f,^;" 65. -right Cheek-Teeth of 

 by a deep internal and, more posteriorly, Mkrotus agresHs (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 m^ 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 Distribution. The 

 other five are British. 



Status : — The arrival of a newer and more vigorous competitor in 



A B 



