﻿32 NORTH AMERICAN FAUNA. ^ £Xo.i2. 



Mamm;e 4. " . 



Size very large - . - . - Xeofiber 



Size medium or smul'. 



Plantar tubercles (i Auteliomys 



Plantar tubercles 5. 



Skull not fattened. - Fedomys 



- - Skull flattened. 



Palat e normal Piiymys 



Palate abnormal Hyperacrias 



8. SUBGEXERA OF MICKOTUS GEOFPED BY ESSENTIAL CHAEACTERS. 



Palate normal. — Microiiis, Fedomys, Fitymys, ChUotus, Fliaiomys, Arvicola, Lagurus, 



Palate abnormal. — Xeojiher, AJiicola, Hyperacrius, Eoihenomys, Anteliomys. 



Third lower molar always with closed triangles. — Xeojiher, Lagurus. 



Third lower molar normally without closed triangles. — Microtus, Fedomys, Fitymys, 

 Chilotus, Fhaiomys, Arvicola, Eoilienomys, Anteliomys, Alticola, Eyperacrius. 



First lower molar normally with 5 closed triangles and 9 salient angles. — Microtus, 

 Chilotus, Xeojiher, Lagurus. 



First lower molar normally with 3 or 4 closed triangles and 9 salient angles — 

 Fedomys, Fitymys, Fhaiomys, Alticola, Hyperacrius. * 



First lower molar normally with 3 closed triangles and 7 salient angles. — Arvicola. 



First lower molar without closed triangles. — Anteliomys, Eothenomys. 



Third upper molar normally with 3 closed triangles and 7 to 8 salient angles. — 

 2Iicrotus. 



Third upper molar normally with 2 closed triangles and 6 salient -Awgle^.—Xeofiher, 

 Arvicola, Fitymys, Fedomys, Fhaiomys, Chilotus. 

 Third upper molar without closed triangles. — Anteliomys, Eothenomys. 

 Mamma? 10. — Fhaiomys. 



Mamma? 8. — Arvicola, Microtus, Alticola, Chilotus, Lagurus. 

 Mamma' 4. — Xeojiher, Fitymys, Fedomys, Anteliomys, Llyperacrius. 

 Plantar tubercles 6. — Microtus, Fhaiomys, Anteliomys, Alticola. 



Plantar tubercles 5. — Xeojiher, Arvicola, Fitymys, Fedomys, Chilotus, Lagurus, Hyper- 

 acrius. 



DESCRIPTIONS OF LIYINO GENERA AND SUBGENERA. 



Genus SYNAPTOMYS Baird. 

 Synaptomys Baird, Mamm. N. Am., p. 558, 1857. Type Synaptomys cooperi Baird. 



Geogrcq^hic distr ihiition of type species.' — Boreal, Transition, and north- 

 ern edge of Upper Austral Zone in eastern Xorth America from the 

 Atlantic coast to Minnesota. 



Geograpli ic distrihiition of genus. — North America from northern edge, 

 of LoAver Austral Zone northward. 



Essential characters : 



Upper incisors with distinct longitudinal grooves. 



Lower incisors with roots on inner (lingual) side of molars. 



Molars rootless. 



Enamel pattern characterized by great depth of reentrant angles on outer side 

 of maxillary teeth and on inner side of mandibular teeth. 



m 1 with three closed triangles and two transverse loops, or with four trans- 

 verse loops and no closed triangles. 



m 3 with four transverse loops and no closed triangles. 



Feet not specially modified. 



Soles and palms with well-develojDed tubercles. 



Thumb with large flattened ligulate nail. 



Tail very slightly longer than hind foot, terete. 



External ear well developed. 



