RODENTIA ARVICOLA. 599 



ARVICOLA, Lacep. 



"Arvicola, LACEPEDB, Tableau, 1803." 



Soles of the feet nearly naked. Tail as long as the hind foot, usually longer, from to f 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 

 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 a valve to the ear in the form of a much devel oped antitragus, by which 

 it can 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. Each 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 Mus, 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. 



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. 



