MURID^ 373 



MURIDiE. 



MICE AND RATS. 



These are small rodents of varied habits and world-wide 

 distribution. They date from the lower Eocene of North 

 America and the lower Oligocene of Europe. 



They have the thumbs rudimentary, the clavicles well 

 developed, the tibiae and fibulae united. The skull has con- 

 tracted frontal bones without post-orbital processes ; slender 

 zygomatic arches in which the short jugal bones are generally 

 reduced to splints between the long zygomatic processes of the 

 maxillae and squamosals ; the lower roots of the former 

 processes more or less flattened into perpendicular plates ; and 

 the infra-orbital foramina tall and wide above, narrow below. 



The dental formula is usually given as : — 



.1-1 o ,o ^-•i , 



I -I o o 3-3 



this covers all, except a few Oriental forms, in which the 

 posterior cheek-teeth disappear ; but Forsyth Major [AtH. Soc. 

 Ital. Sci. Nat., xv., 1872, iii) considered that in th.& Microiincs 

 the anterior cheek-teeth are persistent milk-molars. 



The cheek-teeth are superficially very diverse in structure. 

 They may be high- or low-crowned, rooted or rootless, with 

 prismatic or tubercular crowns of various degrees of complexity. 

 All, however, may be traced to a common origin, a primitive, 

 short-crowned, rooted organ with a highly complicated tubercular 

 grinding surface. Even in the most highly specialised MicrotincB 

 the young unworn cheek-teeth- bear for a short time a tubercular 

 enamel cap suggestive of their ancestry ; and a tubercular 

 tooth may wear down with use to a prismatic arrangement. 



The food varies from grass to flesh, though no species ^ is 

 entirely carnivorous ; hence the Muridce are almost ubiqui- 

 tous, often occur in great numbers, and have probably caused 

 more injury to man than any other mammals. Their destruc- 



' All British mice and rats are at times cannibals, Epimys norvegicus and 

 Evotomys glareolus being probably the most guilty. 



