﻿30 NORTH AMERICAN FAUNA. [Xo.12. 



4. KEY TO SUBGENERA OF SYXAPTOMYS. 



Mandibular molars with closed triangles on onter side Sijnapiomys 



Mandibular molars without closed triangles on outer side Mictomys 



5. KEY TO THE SUBGE^^ERA OF MICROTUS. 



[Based primarily on the bony palate.] 



Pahite normal or nearly so (see p. 27). 



Third lower molar with all triangles closed.. Lagurus 



Third lower molar normally without closed triangles. 

 Claws small, those on front feet always shortest. 



Plantar tubercles 6 Microius 



Plantar tubercles 5. 



Tail more than 30 per cent of total length Arvicola 



Tail less than 30 per cent of total length. 



m 1 with 5 closed triangles Cliilotiis 



m 1 with 3 closed triangles Pedomys 



Claws large, those on front feet usually longest. 



Fur long and soft Fliaiomys 



Fur dense and mole-like Pitymys 



Palate highly abnormal. 



Palate ending in a broad median plate cut off from maxillaries at the sides. • 



Third lower molar with all triangles closed yeofiher 



Third lower molar without closed triangles. 



Skull flat; audital bulla^ small . .' Hyperacrius 



Skull high; audital bullae large AJticola 



Posterior border of palate continuous between maxillaries. 



Posterior border of jialate straight Poflienomys 



Posterior border of jialate with median projfM-tion Anfeliomys 



6. KEY TO THE SUBGENERA OF MICROTUS.' 

 [Based primarily on the teeth.] 

 (jy 1 with 6 or 7 closed triangles.) 

 (Plantar tubercles 5.) 



{Small; not aquatic ; fur short Chilotus) 



(Large; aquatic ; fur long Arvicola) 



(Plantar tubercles 6 Jficrotus) 



m 1 with 6 closed triaBgles. 



m 3 with 3 closed triangles. 



m 3 with triangles always closeu Neofiber 



m 3 with triangles normally open. 

 Plantar tubercles 6. 



Fur not specially modified, claws moderate. 



Posterior loop of m 3 short or strongly curved; palate normal. Microtus 



(Posterior loop of m 3 long and straight ; palate abnormal.) 



(Slcull broad and fat ; plantar tubercles 5 Hyperacrius) 



(Skull not broad and fat ; plantar tubercles 6 Alticola) 



(Fur rery long and soft, aspect lemming-like, claws rery long. Phaiomys) 

 (Plantar tubercles 5.) 



(Small; not aquatic ; fur short Chilotus) 



(Large; aquatic ; fur long Arvicola) 



^ Characters in heavy-faced type are those of specimens with normal enamel pat- 

 tern; characters in italics (inserted in parentheses) are those of specimens with 

 abnormal enamel pattern. 



