MAMMALIA. 283 



being sometimes adapted for cutting the food, but more usual- 

 ly for chewing and grinding it down. 



FIG. 140. Teeth of the right side of the lower jaw of the Chimpanzee (after Owen), i In- 

 cisors ; c Canine tooth ; pm Prseraolars ; m Molars. 



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. 140) would be as 

 follows : 



i 2 ; c 1 ; pm 2 ; m 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 : 



i 2 2 ; c I 1 ; pm 22 ; m 33. 



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 in the upper row referring to the upper jaw, and 

 those in the lower row to the lower jaw; the short dashes be- 

 tween the figures of each row still indicating the teeth on the 

 two sides of the mouth. The complete formula would there- 

 fore run as follows : 



