I 



I 



KEY TO GENERA gg 



forming an inclined septum between the postero- 

 lateral pits (as in Microtvs). 



Upper incisors strongly curved, each with a well- 

 marked anterior groove; second and third transverse 

 loops of w3 separated chiefly by second outer infold, 

 hkull lightly built, long and narrow. Auditory 

 bulliB large and globular, internally with a wide 

 meshwork of bony trabecule. 



General outward form vole-Hke ; ears evident above 

 fur; palms and soles naked between pads; palmar 

 tubercles 4, plantar tubercles 6; claws and thumb- 

 nail not speciaUy enlarged; tail slightly longer than 



hind-foot, rather thinly clad Synaptomys. 



a. Lower molars with closed triangles, their outer 

 infolds and sahent angles well developed. 

 Upper incisors very broad. 



Rostral part of skuU very stout; palate without 

 long posterior spine. 

 Mammae 1-2= 6 . . . Subgenus Synaptomys. 

 b. Lower molars without closed triangles; their outer 

 infolds and salient angles not well developed. 

 Upper incisors not very broad. 

 Rostral part of skull slender; palate with lontr 

 posterior spine. ° 



Mamm^2_2 = 8 . . Subgenus Mictomys. 



ralatal surface of skull terminating behind as a simple * 

 transverse shelf; its median spinous process when 

 present horizontal and ending freely (as in Evotomys). 

 Upper incisors less strongly curved, without well- 

 marked anterior grooves, and with much hollowed 

 tubular worn surfaces; second and third transverse 

 loops of m? separated by first inner and second outer 

 folds. 



Skull relatively massive, broad and depressed 

 Mammte 2—2=8. 

 i. External form vole-Uke, but more thick-set than in 

 Synaptomys; fore-claws and thumb-nail not 

 specially enlarged, and skeleton of hand not 

 specially modified. 



Skull rather lightly built ; zygomata less widely 

 expanded ; squamosals relatively widely separated 

 in front. Auditory bulla? globular . . . Myopus 

 1. External form highly modified for fossorial habits 

 Ears hidden in the fur. Hands large and broad 

 the claws and thumb-nail greatlv enlarged; meta- 

 carpals shortened and ungual phalanges lengthened 

 . for support of claws. Feet short and broad, with 

 large claws. Palms and soles densely haired- 

 palmar and plantar tubercles vestigial. Tail 

 short, robust, densely clothed, and with a long 

 terminal pencil. 



Skull massive, broad and depressed; zygomata very 

 heavy, strong, and widely expanded ;' squamosals 

 more closely approximated in front. Auditory 

 bulls ovate or pyriform, not greatly inflated- 

 densely spongy within Lemmas 



