MAMMALIA. 309 
sometimes wanting, or are sometimes present in one jaw and 
not in the other ; but there are never more than four altogether 
—that is to say, one in each jaw on each side. The premolars 
and molars (fm and wz) are the so-called “ back-teeth,” and they 
Fig. 163.—Teeth of the right side of the lower jaw of the Chimpanzee (after 
Owen). z Incisors; c Canine tooth 3 7 Preemolars; #z Molars. 
vary a good deal in number and function, being sometimes 
adapted for cutting the food, but more usually for chewing and 
grinding it down. 
All these kinds of teeth are not necessarily present, and the 
teeth constitute most important characters for separating the 
various orders of Mammals from one another. For this reason 
it is usual to express the number of the teeth in any particular 
animal by an arithmetical formula, called the dental formula. 
For example, the formula for the portion of the jaw of the 
Chimpanzee figured above (fig. 168) would be as follows :— 
£2 (al 3 Ue (2.2. Hag. 3) 
But this is only one half of the lower jaw, and the dental for- 
mula must include both sides, so that it would be :— 
22—2; CI—1; pm 2—23; m 3—3. 
That this would be the formula is at once evident, when it is 
remembered that the two sides of the jaw of course contain 
exactly the same teeth. Still, the formula as given above only 
includes the lower jaw, and to render it perfect it must take in 
the teeth of the upper jaw as well. This is effected by placing 
the figures in two rows separated by short lines, all the figures 
