Sub-Family ARVICOLINAE. 
Incisors as broad as deep, molars fB§, rootless, or with very short roots composed of alternating triangular prisms. 
Ante-orbital opening moderate. Intermolar portion of the palate descending below the level of the anterior portion ; the 
space between incisors and molars excavated. Descending ramus of the lower jaw situated above the plane of the crowns of 
the molars. 
To the above general characters may be added: ears short and generally hidden in the fur. 
Muzzle broad and rounded. Tail short, less than the body, sometimes not as long as the head, 
mostly clothed thickly with hair. Whiskers (as in Murinae) in five horizontal series. 
The genera hitherto described of this sub-family are Arvicola, (in its most general sense,) 
Myodes, and Fiber, the species of which are confined to the northern half of the globe— Myodes, 
in fact, being the most northern form of Rodents known. All are abundantly represented in 
North America, though Myodes proper has not yet been positively detected within the limits of 
the United States. Fiber is peculiar to this country. 
In external appearance, the Arvicolinae differ from the Murinae in the blunt snout, (not 
pointed,) short ears nearly concealed in the fur, very short and hairy tail, &c. The skull is 
shorter and deeper than in the Murinae, much broader ; greatly compressed in the inter-orbital 
region. The incisors are much broader, sometimes wider than deep. The molars are rootless, 
growing constantly from a persistent pulp ; they are composed of triangular prisms aggregated 
together. 
The genus Neotoma, which has molar teeth similarly constituted, but provided with roots, 
serves to connect closely the Arvicolinae and Murinae, although in all other respects of shape, 
size of eyes, ears, tail, &c., it accords perfectly with the rats. 
In very old individuals of some forms of the Arvicolinae, the supply of new tooth matter 
sometimes diminishes, and finally stops altogether; this is accompanied by the formation of 
irregular fangs, which may, however, be readily distinguished from the genuine fangs of the 
rooted molars, by their irregular form. Genera belonging to other sub-families are liable to the 
same condition, as in the case of the Beaver. 
To mention more fully the characters of the Arvicolinae, it may be stated that the anterior 
root of the zygoma is thrown up from the plane of the palate in the form of a thin plate, 
obliquely placed; its lower edge is emarginated. The malar bone does not form part of the 
outer boundary of the ante-orbital opening, which is rather wider below than in the Murinae, 
and has its upper division bounded below by a lamellar process, bent back from the upper max¬ 
illary bone. The malar bone is broad and vertically compressed, and does not extend back far 
enough to enter into the glenoid cavity. 
The temporal bone is produced anteriorly and laterally, so as, in some species, to form an 
angle. . 
The palate is more or less contracted in front, the inter-molar portion (with the skull in a 
natural position) descending generally below the level of the anterior portion. The palate is 
generally moderately broad, and but slightly contracted between the anterior molars ; it does not 
extend as far back as in the rats, but ceases about opposite the middle of the last upper molars. 
The incisive foramina are large, and situated partly in the maxillary and partly in the inter¬ 
maxillary bones. 
