MURID/= 373 
MURID£. 
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 tibie and fibule 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 
maxilla 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 :— 
pea ¢ a pm Ss m3—3=16; 
I-1I ° ° 3-3 
this covers all, except a few Oriental forms, in which the 
posterior cheek-teeth disappear ; but Forsyth Major (Aé¢z. Soc. 
Ital. Sct. Nat., xv., 1872, 111) considered that in the AZzcrotine 
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 JZccrotene 
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 J/urid@ are almost ubiqui- 
tous, often occur in great numbers, and have probably caused 
more injury to man than any other mammals. Their destruc- 

1 All British mice and rats are at times cannibals, Epzmys norvegicus and 
Evotomys glareolus being probably the most guilty. 
