96 MICEOTIN^ 



absent from cheek-teeth ; cement present 

 in re-entrant folds; m^ without closed 

 triangles. 

 *. External characters normal, not obviously 

 modified for fossorial habits. 

 f. Upper incisors grooved, broad and re- 

 curved; lower incisors short, scarcely 

 invading the condylar process; m^ 

 with three outer and two inner sahent 

 angles; ^n^ with four closed triangles, 

 the fifth triangle confluent with the 

 short anterior loop; m^ with its third 

 outer angle obsolete. 



Skull massive ; temporal ridges fused 

 in the interorbital region; post-orbital 

 processes prominent and peg-like; 

 palate normal. Auditory bullae with 

 spongy tissue within. 



Fur long and coarse ; tail moderately 

 long; plantar tubercles 6. Mammae 



2—2=8 Proedromys. 



tf. Upper incisors normally without grooves, 

 not unusually broad or recurved ; lower 

 incisors long, ascending the condylar 

 process to a greater or less extent. 



Skull with temporal ridges fused in the 

 interorbital region when adult; palate 

 normal. Auditory buUae with spongy 



tissue within Microtus. 



**. External form and cranial characters modi- 

 fied for fossorial life. Skull broad and 

 flat; temporal ridges fusing in inter- 

 orbital region in old age. Auditory bullae 

 densely spongy within; mastoid portion 

 inflated. Ears small, 

 f. Fur short and dense; claws not length- 

 ened; plantar tubercles 5. Mammae 



2-2=8 ChUotus. 



tf. Fur soft and fine ; fore-claws considerably 

 lengthened; soles densely haired; 

 plantar tubercles 6, very small, crowded 

 together, and concealed; tail short; 

 m^ with three saUent angles on each 

 side; m^ with five closed triangles and 

 small squarish anterior loop. 



Lasiopodomys. 

 p. Vestiges of " intermediate tubercles " (" pro- 

 toconule," " metaconule," etc.) normally 

 present in upper molars. Re-entrant folds 

 wide, without cement; mj vidth five closed 

 triangles; m^ with closed triangles. 



SkuU short and broad; temporal ridges 

 salient but separated in the interorbital 

 region by a median sulcus ; squamosals 

 widely separated anteriorly, their post- 

 orbital processes peg-like and prominent; 



