50 



Rats and Mice 



that of each lower incisor forms a smaller segment of a much 

 larger circle and proceeds backwards beneath, or beside, the roots 

 of the cheek-teeth, to terminate in the ascending portion of the 

 mandibular ramus, behind and to a greater or less extent above 

 the level of the molars. The base of each incisor is hollow and 

 open ; in life the cavity lodges the dentinal pulp from which the 

 tooth is developed ; growth is continued throughout Hfe and the 

 tooth is gradually pushed forwards. Under normal conditions, in 

 adults, the growth and forward movement of the incisors take 

 place at a rate which exactly compensates the loss by wear 

 suffered at the working end. Separated from the incisors by a 

 long toothless interval, or diasteme, are the cheek-teeth ; of these 

 there are never more than three on each side above and below, 

 and the anterior one in each jaw is always considerably larger 

 than those behind it. In many forms the posterior tooth is greatly 

 reduced, and in some it is normally absent. The great relative 

 size of the front cheek-teeth is probably due to their position in 

 the jaws, for they are placed just at that point where the com- 

 bined activities of the temporal and masseter muscles produce the 

 greatest pressure between the upper and lower tooth-rows. 



Now the characters described above, with others too numerous 

 to discuss here, are shared by all Muridae ; any animal showing 

 such a combination of characters would be entitled to a place in 

 this family ; and it would in ordinary language be described, 

 according to its size, either as a " rat " or as a mouse." 

 Zoologists, however, tend to restrict still further the use of these 

 names ; they have divided the Muridae into several sub-families, 

 of which the principal are the Microtinae or Voles and Lemmings, 

 the Cricetinac or Hamsters, and the Murinae or true Eats and 

 Mice. The leading character of each of these sub-famiHes is seen 

 in the cheek-teeth,* and for our present purpose they may be 

 sufficiently distinguished as follows :— 



Cheek-teeth with tall prismatic crowns, in most genera per- 

 sistently growing and rootless; their grinding surfaces, 

 except when quite unworn, flat and displaying a pattern of 

 more or less alternating triangles (Figs. 2 D and 4 B-D). 

 Microtinae, or Voles and Lemmings. 



* To examine the molars or cheek-teeth of Muridae in recently caught 

 specimens it is only necessary to cut as deeply as possible into the angles of 

 the mouth with a pair of scissors ; the mouth can then be easily opened. 



