﻿July, 1896. j 



GENUS MICKOTUS. 



45 



Fig. 20.— Side view of molars of 

 adult Microtus alleni. (x 2.) 



brought forward for this purpose prove to be either wholly inconstant 

 or constant only when particular subgenera are held yi view. 



Teeth. — Although the skull of Microtus i)resents no tangible diag- 

 nostic characters, the teeth are readily distinguishable from those of 

 all other members of the subfamily. The upper incisors are never 

 grooved except in occasional abnormal specimens. The root of the 

 lower incisor crosses the line of the molar 

 series between the second and third molars, 

 causing a greater displacement of the roots 

 of the latter (PI. Ill, fig. 3) than occurs in 

 any "other genus. It terminates in the ascend- 

 ing ramus of the mandible at a point slightly 

 above and behind the dental foramen (PI. Ill, 

 fig. 3). The molarSj even in extreme old age, 

 are never rooted (fig. 20). This character 

 alone distinguishes them from the molars of 

 the other voles. The x)attern of enamel folding 

 varies considerably in the different subgenera, and forms one of the 

 numerous characters by which the latter may be separated. Detailed 

 descriptions of the enamel i)atterus are given in the accounts of the 

 subgenera. 



External form. — In external form the members of the genus Microtus 

 vary excessively. Some resemble lemmings so closely that thej^ have 

 been associated with these by certain writers. Others are modified for 

 an aquatic life and in consequence have more the appearance of musk- 

 rats {Fiber). Still others pass most of their time under 

 ground. In these the ears, eyes, and tail are reduced, 

 the front feet enlarged, and the fur so modified as to 

 suggest that of the moles. The great majority of spe- 

 cies, however, show none of these special adaptations, 

 but resemble in a general way the members of the genera 

 Fhenacomys and Evotomys. Whatever maybe the modi- 

 fications in form, the tail is almost invariably longer 

 than the hind foot and the thumb is armed with a small 

 or rudimentary ijointed nail (fig. 21). 

 General remarlcs. — The characters of Microtus, as contrasted with 

 Evotomys and Phenacomys, have already been given (p. 42) and need not 

 be repeated here. 



Subgenus EOTHENOMYSi Miller. 

 New subgeuus. Type Arvicola melavogaster Milne-Edwards. 



Geographic distribution of type species. — Moupin, western Sechuen, 

 and western Fokien, China. (Blanford.) 



Geographic distribution of subgenus. — Microtus melanogaster is the only 

 known species of Eothenomys, hence the geographic distribution of the 

 subgenus is the same as that of the type species. 



Fig. 21.— Left front 

 foot, Microtus 

 terrestris. 



^'H(hi, the morning (eastern); Qsv, from; juvi, mouse. 



