KEY TO GENERA 97 



palate normal. Auditory bullae very large; 

 mastoid portion and legmen tympani swollen ; 

 densely spongy within. 

 Upper incisors strongly curved. 

 External form Lemming-like; fur long 

 and soft; ears very small, concealed in fur; 

 claws moderate, the hind ones slightly the 

 longer ; soles densely haired ; plantar tuber- 

 cles 5, concealed ; tail very short. Mammae 



2—2=8 Lagurus. 



*. External form more Lemming-like; tail 

 shorter; m^ with four closed triangles 

 and three outer saUent angles. 



Subgenus Lagurcs: 

 **. External form less Lemming-like; tail 

 longer; m^ with three closed triangles 

 and two outer salient angles. 



Subgenus Lemmiscus. 

 b. Inguinal mam'rase absent or f unctionless ; rwj with 

 its fourth and fifth triangles indifferently closed 

 or open, the number of closed triangles varying 

 individually between three and five. Re-entrant 

 folds of cheek-teeth deep and narrow, partly filled 

 with cement; m^ with closed triangles, its first 

 outer fold deep. 



Skull with temporal ridges fused in the inter- 

 orbital region when adult. Auditory bullae with a 

 small amount of spongy tissue witliin ; stapedial 

 artery enclosed in bony tube. 



Fur long and soft ; ears large ; plantar tubercles 5. 



1. Skull long, narrow, and somewhat depressed. 



Auditory bullae small. Cheek-teeth, particularly 

 the upper molars, like those of Phenacomys in 

 general appearance ; tn^ with three saUent angles 

 on each side; mj with three or four outer and 

 four inner salient angles; m^ with three salient 

 angles on each side. Tail long, half length of 

 head and body. Mammae 2—0=4. 



Orthriomys. 



2. Skull less depressed. Auditory bullae large. 



Cheek-teeth of more normal general appearance; 



m^ with three outer and four inner salient angles ; 



wij with four outer and five inner salient angles ; 



m^ with two outer and three inner salient angles. 



Tail shorter, about one-third length of head and 



body. Mammaj 2—1 = 6 . . . Herpetomys. 

 n. m§ extremely reduced ; m* always and m^ sometimes con- 

 spicuously smaller than m% ; implanted portion of m^ 

 sharply recurved, lying upon and tending to be con- 

 centric with the lingual aspect of the dorsal surface of the 

 alveolus of the lower incisor. 



Cheek-teeth rooted in adults ; re-entrant folds wide and 

 shallow, without cement ; inner and outer dentinal spaces 

 mostly confluent, forming or tending to form transverse 

 pairs; enamel thick not clearly diUerentiated ; enamel 

 pattern simplified, that of m^, vi-, and m^ essentially normal. 

 V.L. " H 



