RODENTIA—ARVICOLA. 509 
ARVICOLA, Lacep. 
‘‘Arvicola, LacerepE, Tableau, 1803.’’ 
Soles of the feet nearly naked. ‘Tail as long as the hind foot, usually longer, from } to 3 the length of the body ; cylin- 
drical, or nearly so; well covered with hairs. The posterior upper molar composed of five or six prisms; the posterior 
lower one of three; its re-entrant angles opposite to each other. 
The above characters will serve to separate the Arvicolas from Fiber and Myodes, although. 
there is abundant room to make several subdivisions. The species of the genus are distributed., 
. . . . ‘Bs 
over the northern hemisphere of America, Europe, and Asia, none, as yet, having been detected 
in South America or Africa, as, indeed, is the case with the whole family. Many species are 
eminently aquatic, and have avalve to the ear in the form of a much developed antitragus, by which 
itcan be closed when under water ; others live in dry pastures, or high lands, where they fre- 
quently do great mischief by their consumption of grain and fruit, or by destroying young trees 
in eating off the bark. 
The upper incisors are stouter than the lower, the latter much rounded. The molars decrease 
from before behind, and the lines of their outer edges at the same time approximate behind so 
as nearly to intersect. Hach is composed of triangular prisms placed in two alternating series, 
so that the edges have.a zigzag or serrate appearance. These prisms vary somewhat with the 
species, and afford excellent specific characters. The molars are all rootless, except in one form 
represented in this country by Arvicola gapperi. 
The skull differs strikingly from that of Mus in being much shorter and broader, with a sudden 
contraction of the inter-orbital region of the frontal bones. The zygomatic arches are much 
shorter, and stand more apart. The ante-orbital opening at the anterior root of this arch is 
rather broader than in Jus, and the thin lamellar plate forming its outer border, instead of 
projecting considerably beyond the junction of the upper anterior branch of the zygomatic arch 
with the frontal bone, is emarginated concavely just below this region, not extending, in fact, 
beyond the naso-frontal suture. 
The palate presents considerable differences, some of which have already been adverted to. 
The stomach is provided with a strong constriction, dividing it into two halves ; on the right 
one there is an additional swelling, like a third stomach, The coecum is large, with many 
constrictions. The colon is twisted spirally for a considerable part of its course. The gall 
bladder is generally present. The end of the rectum is surrounded by a gland, which discharges 
a fetid fluid into it, near the anus. © 2 
The testicles, prostate, and Cowper’s glands have an enormous development, especially during 
the rutting season. 
The genus Arvicola, or field mice, embraces many species, spread over the northern hemi- 
sphere of America, Europe, and Asia, and in the number of individuals probably far exceeds any 
other among mammals. Distributed uniformly throughout the countries enumerated, some 
species inhabit the vicinity of the water, others occupy the high lands, while others again fre- 
quent rocky elevated regions, or even Alpine mountain heights. The thick moss and sphag- 
nous swamps of the regions in the vicinity of the arctic circle are said to abound in species both 
of Arvicola and Myodes to an extraordinary degree, and to furnish the starting points of the 
armies of Lemmings which play so conspicuous a feature in the natural history of the north, 
