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U. S. P. R. R. EXP. AND SURVEYS—ZOOLOGY—GENERAL REPORT. 
In more southern countries the field mice sometimes multiply and swarm to an enormous 
extent, appearing suddenly in regions where hut few were previously supposed to exist. In 
America the injury to young trees and shrubs from the ravages of field mice has been some¬ 
times very great; hut it is of European species that the most extraordinary accounts are given 
as to numbers and destructiveness. Thus, in the year 1837, four-fifths of the entire harvest in 
the province of Piombina, in Italy, were devastated by the field mice, which had been driven to 
the high grounds by heavy floods in the meadows. In a single province in Germany, in 1822, 
1,570,000 mice were captured in fourteen days, as shown by official reports. 
As will be seen hereafter, there is good reason to concur with Keyserling and Blasius in 
separating a genus, or, at least, sub-genus Hypadaeus 1 from Arvicola. The former differs 
little externally from Arvicola, except, perhaps, in the more prominent ears, and some other 
minor points. In all, however, the head is short and blunt, the body stout, sometimes very 
much so. The upper lip is bifid, and a shallow groove extends to the nose and passes between 
the naked nostrils, which swell out a little on each side. This groove, when the skin is stretched 
apart, is seen to be naked ; but in the ordinary condition, the region between the nostril and the 
lip appears to be densely furred ; the hair also extends to the extreme end of the muzzle, leaving 
only a naked space around the nostrils. The lips are tumid and hairy to within the mouth. 
The bristles are in five horizontal series. The ears are variable in size, usually concealed in 
the fur ; the surfaces covered with hair, or nearly naked. The eyes are very small. 
The fur is generally long and loose, sometimes short and compact. It is always plumbeous 
at the base. 
The thumb is obsolete externally, or very rudimentary, the corresponding claw almost 
springing directly from the skin. It is variable in size and shape ; sometimes conical, sometimes 
flat, always inconspicuous. The third and fourth fingers are longest; the second and fifth 
shorter. The palm is perfectly naked. The fore feet are half as long, or more, than the hinder. 
The hind foot has the sole hairy for one-half or one-third its length from the heel, or as far as the 
tubercles. The first and fifth toes are inserted on much shorter metatarsals than the second, 
third, and fourth, which are longest. 
The tail is variable, sometimes nearly half as long as the head and body ; more usually from 
one-third to one-fourth, or even less ; never shorter than the hind foot, as in the Lemmings. It 
is sparingly or densely covered with stiff hairs. 
In Hypadaeus, in addition to the characters embraced in the preceding description, the 
general form is slenderer than in most Arvicolae, and the appearance more like that of Sigmodon. 
The size is small; the head rather narrower and pointed. The ears are very large, projecting 
far beyond the fur, and well covered with short, close hairs, which have almost the finish of the 
squirrel’s ear. The antitragus is large. The feet are small, the anterior more than half as 
long as the posterior, and with naked palms and small thumb claw. The sole is hairy for the 
'The genus Arvicola was established by Lacepede in 1803, according to tho Nomenclater Zoologicus of Agassiz. I have not 
been able to ascertain upon what particular species the genus was based ; but it was applied by him both to A. amphibia and 
A. arvalis, types of two very different sections. In 1811, Uliger,' in the Prodromus Syst. Mammalium et Avium, made the 
genus Hypudaevs, and included in it as types the Mus lemmus, amphibius, and arvalis, or nearly the same as Arvicola. The 
genus Lemmus was proposed by Linck, and was of still greater extent as defined by Fischer. These genera were generally 
considered as synonymous until Keyserling and Blasius, in Wirbelthiere Europas, 1842, defined two sub-genera, Arvicola and 
Hypudaeus, and restricted the latter to the species with partly rooted molars, having Arvicola glareola as the type. Not having 
the means of reference to the works of Lacepede and Linck, 1 am unable to say whether Lemmus should be used in preference 
to Hypudaeus, or whether it belongs more strictly to tho true Lemmings, and in this case whether or not it supersedes Myodes 
of Pallas. 
