MICROTUS. 170 



'reiramert'doii. Rhoads, Proc. Acad. Nat. Scien. Phil., 1S94, 

 p. 2S2. 



Aulacomys. Rhoads, Am. Nat., 1894, p. 182. 



Orthriomys. Merr., Proc. Biol. Soc. Wash., 1898, p. 107. 



Herpetomys. Merr., Proc. Biol. Soc. Wash., 1898, p. 107. 



Molars not rooted, mandibular molars without closed triangles 

 on outer side; upper incisors not grooved; root of lower incisor 

 extending back to third molar, displacing base of that tooth, and 

 terminating above the dental foramen of the ascending ramus. Tail 

 terete, longer than hind foot; posterior border of bony palate very 

 variable; middle part of zygoma only slightly expanded; postorbital 

 process of squamosal shelf- like. 



A. Microtns. 

 Palate normal; lower third molar without closed triangles; lower 

 first molar normally with five closed triangles and nine salient angles; 

 upper third molar normally with three closed triangles and seven or 

 eight salient angles. Upper incisors not grooved. Mamm;i' eight, 

 four pectoral and four inguinal. Plant;r tubercles six; soles moder- 

 ately hairy. 



Fig. 38. Microtus pennsylvanicus. 



No. 424 Field Columbian Museum Coll. Nat. size. 



LOWKR TliOTH KOW. UlTER TOOTH ROW. 



KnlarRed t times. Enlarged 5 times. 



256. pennsylvanicus. ( .lA/y), Ord, Guthr. Geog., 1815, 2d ed., 

 p. 292. 

 pratcnsts, Rafin., Am. Month. Mag., 1817, 11, p. 45. 

 no'i'eboracensis, Rafin., Ann. Nat., 1820, p. 3. 

 paliistris, Harl., Faun. Amer., 1825, p. 136. 



ripariiis, Ord, Journ. Acad. Nat. Scien. Phila., 1825, Iv, p. 305. 

 albo-rufescens, Emmons, Rep. Quad. Mass., 1840, p. 60. 

 hirsutus, Emmons, Rep. Quad. Mass., 1840, p. 60. 



