154. Tail almost the same color above and below; usually under twelve inches 



in total length; N. M. into Cal. 



7S[eotoma lepida Thomas Desert Wood Rat 

 Tail much lighter below; often over twelve inches long 155. 



155. White fur on throat, breast and groin lead colored at base; southward 



from Colo. 



J-leotoma mexicana Baird Mexican Wood Rat 

 White fur on these regions white at base 156. 



156. Tail grayish-brown above; Colo, and Kansas southward through N. M. 



and Texas 



J^leotoma rmcropus Baird Texas Wood Rat 

 Tail blackish above; Colo, to Texas, and westward to Cal. 



J^eotoma alhigula Hartley White-throated Wood Rat 



157. Front of upper incisors each with a lengthwise groove near the outer 



edge Lemming Mice 158. 



Upper incisors not grooved 159. 



158. With a few brightly colored hairs at the base of each ear; with eight nip' 



pies; last two lower molars on each side each with three closed loops or 

 triangles; northern New England and Washington state 



Synaptomys horealis (Richardson) Canadian Lemming Mouse 

 No light colored hairs at the base of the ears; with six nipples; last two 

 lower molars on each side each with four closed loops or triangles; 

 eastern and central states 



Synaptomys cooperi Baird Lemming Mouse 



159. With a reddish dorsal band from the crown of the head to the root of 



the tail; molar teeth each with two small roots in adult specimens 

 Red-backed Mice 160. 



Without a reddish dorsal band 162. 



160. In the northern states and southward in the mountains; dorsal band 



usually well defined 



Clethrionomys gapperi (Vigors) Capper Red-backed Mouse 

 (Evotomys gapperi (Vigors) ) 

 In the Pacific states; dorsal band merging into the color of the sides 161. 



161. Tail about one-third total length; living on the ground 



Clethrionomys califomicus Merriam California Red-backed 

 Mouse 

 Tail longer; living in trees 164. 



162. Grinding teeth with pronged roots; Pacific States 163. 

 Grinding teeth without pronged roots; generally distributed 165. 



163. Tail less than one-fourth total length; u£per parts grayish; Pacific states 



Phenacomys intermedins Merriam Mountain Lemming Mouse 



Tail over one-third total length; upper parts brownish; in Oregon and 



Cal. 164. 



164. Upper parts bright reddish-brown 



Phenacomys longicaudus True Red Tree Mouse 

 Upper parts brown, mixed with black 



Phenacomys albipes Merriam Coastal Lemming Mouse 



521 



