FAMILY. 



MURIDAE. 



3 3 



Incisors -j-; molars 55 to ^-^, usually j- 5, rooted or rootless. A conspicuous opening in the zygomatic process of the 

 upper jaw, which is generally a vertical slit widened above, hut sometimes a rounded aperture, as in 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 : 



DIPODINAE. Molars unequal, generally rooted. Ante-orbital foramen very large ; rounded or 

 oval ; bounded externally in part by the malar. Hind legs greatly elongated. 



MURINAE. 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 : 



SPALACINAE. 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 



