HOW ANIMALS EAT. 69 
ly correspond with the structure and habits of the animal, 
so that a single tooth is sufficent to indicate the mode of 
life and to identify the species.” In the Ruminants, Ro- 
dents, Horses, and Elephants, the summits of the molars 
are flat, like mill-stones, with transverse or curving ridges 
of enamel. In the Cats and Dogs, they are narrow and 
sharp, passing by each other like the blades of scissors, 
and therefore cutting, rather than grinding, the food. 
The more purely carnivorous the species, and the more 
it feeds upon living prey, the fewer the molars. In ani- 
mals living on mixed diet, as the Hog and Man, the 
crowns have blunt tubercles. Premolars, or bicuspids, 
are those which were preceded by milk-teeth; the true, 
or back, molars had no predecessors. 
The dentition of Mammals is expressed by a formula, 
which is a combination of initial letters and figures in 
Fre. 33.—Teeth of the right lower jaw of adult male Chimpanzee (Troglodytes niger), 
natural size. The molar series does not form a curve, as in Man. 
fractional form, to show the number and kind of teeth 
on each side of both jaws. Thus, the formula for Man 
eerie = it, 22, SO 
emails Pig as 20) a= = oes 
The teeth of Mammals are always restricted to the mar- 
gins of the jaws, and form a single row in each. But they 
never form an unbroken series in any living species, ex- 
