MURID^ 99 



Family Muridse. Rats and Mice. 



Skull much contracted interorbitally ; aniteorbital foramen 

 large, wide in its upper part, narrow at bottomi; zygomatic arch 

 spi-eading, slender, the maxillar part prolonged backward and the 

 malar correspondingly diminished; no premolars; molars rooted 

 or rootless, tuberculate or with angular enamel folds on grind- 

 ing surface; no external cheek pouches; internal cheek pouches 

 sometimes present; clavicles present; tibia and fibula united in 

 their lower parts; inner toe of front foot rudimentary. 



This is a large family of nearly fifty genera and probably 

 five hundred species divided among several subfamilies. The 

 family is represented in all parts of the world, but each family 

 preponderates in some particular zoo-geographical region. 



Few members of this family are utilized by mankind as 

 food. Taken as a whole it may be classed as noxious through 

 their destroying considerable amounts of cultivated or indigenous 

 crops or their stored products. Rats and Mice are more or less 

 omnivorous. Perhaps their largest item of food is seeds, but 

 scarcely anything edible comes amiss to some or another of the 

 species. 



Most of the species are nocturnal. The modes of life are 

 varied; some are semiaquatic; a few are semiarboreal ; most 

 species are terrestrial and again others are more or less sub- 

 terranean. The sexes are practically alike; the young are usually 

 darker than the adults; distinct seasonal changes are few. 



Dental formula, I, i — i ; C, o — o; P, o — o; M, 3 — 3X2^16. 



Subfamily Murinae. 



Skull long and narrow*; rostrum long; nasals projecting be- 

 yond premaxillaries ; enlargement at root of lower incisor near 

 base of condylar process greatest on the outer surface ; tip or 

 angular process below the plane of the summits of lower molars ; 

 notch between tip of angular process of lower jaw and condylt 

 shallow ; molars rooted, tuberculate, with tubercles in three series ; 

 palate extending further back than molars. 



