﻿44 



NORTH AMERICAN FAUNA. 



[No. 12. 



Ill the number and arrangement of triangles the enamel pattern 

 (fl^. 19) is the same as that of the tetramerocTont species of Microtus 

 (see p. 65). The salient angles are, however, for the most i^art rounded, 

 and so placed that the triangles are seldom fully closed. 



External form. — In external form JEvoiomys does not differ essentially 

 from Microtus, although the ears are usually larger. The red or rufous 

 color of most of tlie si)ecies gives them a very different appearance 

 from the other voles. 



General remarks. — The characters which separate Uvotomys from Mi- 

 erotus and Fheiiacomys have been presented in such detail under the lat- 

 ter that it is unnecessary to consider them further. The peculiar bony 

 palate of Evotomys has been considered one of the best generic charac. 

 ters. Since the discovery that it is perfectly reproduced in two subgenera 

 of Microtus {Anteliomys and Eotlienomys) it loses much of its importance. 



The genus Evotomys is represented in Europe, Asia, and North 

 America by numerous species and subspecies Avhose interrelationships 

 are at present little understood. Among the American species may be 

 mentioned E. gapperi (Vigors), E. fuscodorsalis Allen, E. galei Merriam, 

 E. idahoensis Merriam, E. californicus Merriam, and E. occidentalis Mer- 

 riam; among those found in the Old World are E. rutilus (Pallas), E. 

 glareolus (Schreber), and E. rufocanus (Sundevall). 



Genus MICROTUS Schraiik. 



1798. Microtus Schrank, Fauna Boica, I, Iste Abtli., p. 72, 1798. Type by elimination 



Microtus terrestris Sclirank = l/«s arvalis Pall. 

 1883. Microtus Lataste, Le Naturaliste, Tome II, p. 348, 1883. 



1801. Arvicola Lacdpede, Mem. de I'Institnt, III, p. 489, 1801. Type 'Arvicola amplii- 

 Mus^ -=Mns terrestris Linn. 



Geographic distribution of type species. — Central Europe and parts of 

 Asia. 



Geographic distribution of genus. — In both hemispheres the genus 

 Microtus ranges from near the northern limit of mammalian life to the 

 edge of the tropics. 



Essentia l characters : 



Upper incisors without grooves. 



Lower incisors with roots on outer side of molar series. 

 Molars rootless. 



Enamel pattern characterized by approximate equality of reentrant angles. 



m 1 usually with five closed or nearly closed triangles. 



m 3 with one, two, or three closed triangles. 



Tail nearly always longer than hind foot, terete. 



Feet, fur, eyes, and ears very variable. 



Thumb never with a well-developed ligulate nail. 



S'kull. — The skull of Microtus varies greatly in shape among the dif- 

 ferent subgenera. Full descriptions will be given under each of these. 

 Considering the genus at large it is difficult to frame any diagnosis by 

 which the skull may be in every case distinguished from that of the 

 other voles. Most of the characters which at various times have been 



