90 MICROTIN^ 



GeOUP MlCEOTI. 



Lower incisor long, passing from the lingual to the labial side 

 of the molars between the bases or roots of m^ and WI3, and 

 ascending for a greater or less distance behind the molars 

 to terminate within or near the condylar process. 

 I. m§ not extremely reduced, never conspicuously smaller than 

 mf ; implanted portion of m^ not sharply recurved and 

 never approximately concentric with the alveolus of the 

 lower incisor. 

 A. Skull with palate terminating behind as a transverse shelf, 

 with or without a median spinous process; the latter 

 when present free, its tip never connected with the 

 inner borders of the postero-lateral palatal pits. 

 Auditory bullae without spongy tissue. ^ 

 1. External characters never modified for aquatic habits; 

 size small or medium. 



Skull with the temporal ridges usually separated in 

 the interorbital region of the adult (Hyperacrius 

 excepted). Auditory biiUse thin- walled; stapedial 

 artery naked as it passes through stapes. 



Dentinal spaces of cheek-teeth not always substan- 

 tially closed; m^ never with closed triangles. 



a. Cheek-teeth rooted in adults; m^ with four outer 



and five inner angles. 

 External form normal. 

 Mammae 2—2=8 Evotomys. 



1. Cheek-teeth relatively weak; dentinal spaces of 



jrtj and m, more or less confluent in middle Ufe; 

 m^ not encapsulated or noticeably displaced by 

 the shaft of the incisor. 



SkuU not exceptionally massive or angular. 



Normal Evotomys. 



2. Cheek-teeth robust; dentinal spaces of ?Wi and m^ 



tending to be tightly closed in middle life; 

 m^ noticeably displaced by the shaft of the incisor 

 and encapsulated on the lingual side of the jaw. 

 SkuU massive and angular, resembling that of 

 Microtus in general appearance . rufocanus group.^ 



b. Cheek-teeth rootless and persistently growing. 



1. External form Lemmuig-Uke. Cheek-teeth tall- 



crowned but Ught, in pattern resembling those 

 of normal Evotomys; m^ displaced by the shaft 

 of the incisor and encapsulated . . Aschizomys.' 



2. External form essentially normal, not Lemming- 



hke. Lower molars nearly as in normal Evotomys. 

 a. Skull with the temporal ridges not fused in the 

 interorbital region; post- orbital crests of 

 squamosals never very long; anterior palatal 

 foramina not greatly reduced. Auditory bullae 

 never very small. Re-entrant folds of the 

 cheek-teeth partly filled with cement. External 



^ The only exception to this is afforded by Evotomys kennardi, a fossil 

 species from the Pleistocene of Britain. The bullae of this form contain a 

 little spongy tissue. 



^ Owing to the existence of intermediate forms it is not possible to 

 separate the rufocanus group subgenerically from normal Evotomys. 



3 As to the validity of Aschizomys see pp. 43, 279. 



