MDBID^— GENERAL CONSIDERATIONS. 



3 



be added. The elaboration of the genera which is to follow renders this, 

 however, in a measure superfluous. We may here only allude, then, to the 

 broad general distinctions — the Murince being animals which include, and, in 

 all their members, recall, the familiar house-rat and house-mouse, of lithe and 

 supple form, quick in movement, having large bright eyes, pointed mobile 

 snout, prominent ears, and lengthened limbs and tail ; all of which points are 

 contrasted with the squat and heavy shape, the slower action, very small eyes, 

 blunt snout, inconspicuous ears, and shortened members of the Arvicolince. 

 Tiiere is also a striking difference in the ensemble of the dentition of the two 

 groups — tlie compressed and comparatively small incisors of Murince as 

 against tiie large broad fore teeth of the Arvicolince — the small, tubercular, and 

 rooted molars of the former, in contrast witli the great, flat-topped, prismatic, 

 and continually-growing grinders of the latter, in which it would seem that 

 the perfection of rodent dentition is attained. 



In their geographical distribution in America, moreover, the two groups, 

 though associated to some extent, differ decidedly. The Arvicolince occupy 

 the northern half of the hemisphere ; they are not known to occur farther 

 south than Mexico; they are most numerously represented by individuals in 

 liigh latitudes, while some of them are among the most arctic of mammals. 

 The northern limit of the Murince is, perhaps, not exactly knowM; but they 

 do not reach the arctic seas, v/hile they are most abundant in temperate and 

 warm climates, and spread over Cetitral and South as well as most parts 

 of North America. 



In the following table, prepared with much care, the cranial and dental 

 cliaracters of Muridce, as represented in North America, are exhibited. The 

 points brougijt out, it will be observed, are of varying grade, from those that 

 run through both subfamilies down to those that particularize the subgenera, 

 and serve to indicate the several degrees of relationship which subsist between 

 the respective groups. 



