88 MICROTINiE 



genus in the Pleistocene or even at the present day in North Africa 

 would, of course, be in complete harmony with what is now known 

 about the faunistic relations of North Africa with South-eastern 

 and Central Asia. 



e. Key to the Genera of Microtin^. 



Group Lemivii. 



Lower incisor short, wholly lingual to the molars, and terminating 

 posteriorly in the horizontal ramus opposite or in front of 

 the alveolus of vi^. 



I. Cheek-teeth rooted in adults. 



(None yet discovered.) 



II. Cheek-teeth rootless and persistently growing, with well- 



clifferentiated enamel. 



A. Cheek-teeth longitudinally complex; with the inner and 



outer salient angles and re-entrant folds approximately 

 equal in size or depth ; re-entrant folds without cement ; 

 ml with four sahent angles on each side; m^ with 

 four outer and three inner saUent angles; postero- 

 external angle (" cusp n ") in m^ and m^ much smaller 

 than the other salient angles; m^ with at least five 

 outer and six inner saUent angles, and with at least 

 seven closed triangles in front of its posterior loop. 



Skull with prominent peg-Hke post-orbital processes ; 

 temporal ridges separated in interorbital region by a 

 median sulcus. Auditory bullae with dense internal 

 sponge of bone. 



Ears vestigial and concealed. Hands and feet short 

 and broad ; palms and soles densely clad with crisp hair ; 

 palmar and plantar pads vestigial; fore-claws greatly 

 enlarged and apparently double in winter; thumb 

 with minute nail. Tail shorter than foot, densely 

 clothed, with a long terminal pencil. Mammae 

 2—2=8 Dicrostonyx. 



B. Cheek-teeth longitudinally simpUfied ; inner saUent angles 



of upper, and outer salient angles of lower molars 

 smaller than those of opposite sides; re-entrant folds 

 with cement; m^ with three saUent angles on each 

 side ; m^ with three outer and two inner saUent angles ; 

 rn? apparently consisting of four transverse loops; 

 wij with three outer and four inner saUent angles and 

 with three closed triangles only in front of the posterior 

 loop. Molars broad-crowned; some of the inner 

 saUent angles in upper teeth and of the outer saUent 

 angles in lower teeth presenting an appearance of 

 square truncation. 



Skull with shelf -like post-orbital crests; temporal 

 ridges fusing sooner or later in the interorbital region. 



Thumb with large flattened, strap-shaped nail. 

 1. Palatal surface of skull not terminating posteriorly as a 



simple transverse shelf; its median spinous process 



