238 BRITISH MAMMALS 



which had shrunk to pygmy forms, the elephants no larger than 

 sheep and the hippopotamus the size of a small pig. 



The dormouse, strange to say, does not seem to have been 

 of Asiatic orgin, but to have originated in Africa, where three 

 genera are represented at the present day. The range of the 

 family at present extends from South Africa to Southern Sweden, 

 and from West Africa to Little Russia. 



FAMILY: MURID.E. THE RATS AND MICE 

 The Muridse differ from the Rodents hitherto described in 

 being still further specialised. Their teeth, besides the single 



3rd premolar, 

 often missing. 



j5K ^ --- 



4th premolar 



Molar i. 



Istmolar ^Rmk fc^ Molar 2. "S&kv Molar , 



and molar arAtuiaj . .^^feu __ Molar 



Molar 3. 



TPnaiA am^Kf ^c^i>^j 



3rd molar. . . 



Squirrel. Rat. Water Vole (Microtus). 



MOLAR TEETH OF SQUIRREL, RAT, AND WATER VOLE (Microtus) FOR COMPARISON. 



Twice natural size. 



pair of incisors in each jaw, are reduced to only three pairs of 

 molars above and below. They have lost the premolars. The molar 

 teeth of the Rats are sometimes rooted and sometimes rootless. 

 In the True Mice and their allies they are marked with rubbed- 

 down tubercles (like little craters), which are widening out into 

 ridges of enamel. In other forms these enamel ridges become 

 the usual zigzag, raised, flattened folds. The tail is nearly 

 always hairless, with the skin covered by minute scales. The 

 ears are large and rounded. There are five toes on all four 

 feet, but the first finger (equivalent to our thumb) on the fore 

 feet is rudimentary, and reduced to little more than a nail. It 

 is noteworthy that the thumb should be marked in the True Rats 

 and Mice by a short nail instead of a long claw. 



