FAMILY. 
MURIDAE. 
Incisors =; molars to as usually a, rooted or rootless. A conspicuous opening in the zygomatic process of the 
upper jaw, which is generally a vertical slit widened above, but sometimes a rounded aperture, asin Dipodinae. The coronoid 
and condyloid processes and descending ramus distinct and well developed, situated nearly in the same plane ; the latter 
more or less twisted. - Tibia and fibula united below. , 
The Muridae, as above defined, are spread widely over the globe, and embrace more species 
than any other family of the Rodentia. None are of very large size, Fiber, among the largest, 
while the smallest quadrupeds known, next to the Shrews, are found here. Many of the species 
are cosmopolite, and accompany man in his migrations, and are capable of existing under almost 
the same extremes of climate and external conditions. 
Exclusive of the Dipodinae, which are somewhat aberrant, and may possibly be differently 
placed with propriety, or even erected into a distinct family, the Muridae constitute a very 
natural group. Even the Dipodinae, however, agree in most essential points, a principal differ- 
ence being in the large and rounded opening in the zygomatic process of the upper jaw. This, 
in all the others, consists of a narrow vertical fissure anterior to the corner of the frontal bone, 
widening above, and bounded externally by the zygomatic branch of the upper maxillary, 
which, instead of standing out more or less horizontally, is bent up, so that its anterior edge, 
at least, is almost in a vertical plane, and parallel with its fellow on the opposite side. In fact 
the zygomatic process is divided into three parts, one articulating behind with the malar bone, 
another completing the enclosure of the foramen just described, and a third articulating with 
the ante-orbital process of the frontal. The inferior narrow part of the ante-orbital foramen 
serves for the passage of the infra-orbital nerve ; the wider upper portion, the outlet of which 
is directed upwards, accommodates a portion of the masseter muscle. The suture of the malar 
bone, with the zygomatic process of the upper maxillary, is distinctly visible ; the bone itself 
does not extend to the frontal bone, although in some Dipodinae it reaches the lachrymal. 
The three families into which the Muridae are divided, Dipodinae, Murinae, and Arvicolinae, 
have each their representatives in North America; their characters may be presented, briefly, 
as follows : 
Drroprnaz.—Molars unequal, generally rooted. Ante-orbital foramen very large ; rounded or 
oval; bounded externally in part by the malar. Hind legs greatly elongated. 
MortNat.—Molars rooted, unequal. Ante-orbital foramen a narrow slit expanding above, not 
bounded externally by the malar. Hind legs moderate. 
ARVICOLINAE.—Molars rootless, very rarely with short roots; ante-orbital foramen as in the 
preceding. Hind legs short. 
To these it will probably be necessary to add a fourth, to accommodate certain European 
genera, some of them of comparatively recent discovery, as Ommatostergus, Spalax, and Chto- 
noergus, Which may be characterized as follows: 
SpaLActnaE.—Fore feet with five toes ; the thumb short, but distinct, with a distinct nail. 
Soles covered by stiff long hairs directed outwards. Hind legs scarcely longer than the fore 
