84 PROCEEDINGS: BOSTON SOCIETY NATURAL HISTORY. 
species of Lagurus 4 Fur coarse and harsh. Skull larger than that 
of M. pennsylvanicus , the brain case longer, narrower, and less 
highly arched. 
Color. — Adult in full winter pelage ( 9 No. 2107 collection of 
G. S. Miller, Jr.,— South Point Island, Mass., December 29, 1892). 
General color light gray throughout, purest (almost white) on the 
belly and tinged with wood-brown on the sides and back, the latter 
somewhat darkened by a sprinkling of longer blackish hairs. Dorsal 
surface of feet concolor with belly. Tail indistinctly bicolor, brown¬ 
ish dorsally, whitish ventrally. Whiskers moderately long, reaching 
about to ears, blackish or silvery white according to light. Basal 
four-fifths of fur everywhere slaty plumbeous. 
Skull and teeth. — The skull of Microtus breweri (Plate 1, 
fig. 1), is larger than that of typical M. pennsylvanicus (Plate 1, 
figs. 2 and 3) and has the brain case and especially the interparietal 
of a distinctly different form. When viewed from above, the brain 
case is noticeably longer and narrower than in M. pennsylvanicus. 
The occiput is at the same time depressed and widened, while the 
zygomatic arches are more abruptly flaring. The interparietal is 
longer in its anteroposterior diameter and shorter in its transverse 
diameter than in M. pennsylvanicus. These differences are well 
brought out in the figures. The pattern of enamel folding is the 
same as that of M. pennsylvanicus. 
Color variation due to age and season is not very considerable. 
In summer, while the color does not differ appreciably from that of 
the winter specimens, the fur has the peculiar harshness greatly 
accentuated. The tail is often very indistinctly bicolor and in many 
specimens almost naked. 
The young when half grown or less are usually a shade darker 
than the adults. Such immature specimens have the dusky feet 
and nearly unicolored tail characteristic of the young of most species 
of Microtus. 
In color and quality of the fur Microtus breweri differs so consid¬ 
erably from the common M. pennsylvanicus of the mainland and 
the neighboring islands that a detailed comparison is hardly neces¬ 
sary. The common vole of the Eastern United States varies consid- 
%j 
erably in color and the variations are to a certain degree correlated 
with the physical conditions to which the species is exposed in differ- 
1 For an account of the occurrence of the subgenus Lagurus in America, see Merriam, 
Amer. Nat., 29, p. 758, Aug., 1895. 
