RODENT CHARACTERISTICS 
each cheek, which meet and form a barrier against the passage into the throat 
of chips and dust dislodged in cutting wood or digging in the ground. 
The back teeth have a similar growth in certain kinds, but 
more often are rooted and grow only for a limited period like 
ordinary teeth ; “are quite straight and perpendicular, and 
their crowns, worn flat by mutual attrition, present an in- 
tricate and endlessly varied pattern of the enamel folds,” use- 
ful in distinguishing the diffcS@iiuggggiggs and species.g@NQ 
rodent has more than three molars, but the pre 
present vary in number. The brain is primitive in type, bu 
in the larger rodents shows furrows in proportion to the ani- 
mal’s size, according to the general rule that large creatures 
have brains with more surface convolutions than little ones; 
an odd exception, however, is the beaver, which, although 
comparatively big, and manifesting superior intelligence, has 
a remarkably smooth brain. 
The rodents embrace an 
enormous number of genera 
and species. ‘They are dis- 
tributed all over the world, 
including the Australian re- 
gion, and, being small and 
often nocturnal, and by no Showing how fe cheeks close in behind 
€ Incisors. 
means particular in their diet, 
have managed to thrive and multiply to a greater extent 
than any other group of living mammals. They are chiefly 
terrestrial creatures, and -often burrow or live in ready-made 
burrows. Some, however, such as the voles, are aquatic; 
others, e.g. the squirrels, are arboreal, and there are ‘flying’ 
rodents exemplified by the genus Anomalurus.” It is evi- 
dently needful to sort this immense and diversified assem- 
blage into natural groups in order to examine it. The 
incisor teeth, which form the most characteristic feature of 
405 
