400 GENERAL ZOOLOGY 



are shorter, broader, and flattened on top, and are fitted for 

 grinding (Fig. 211). The incisors are four in number, two 

 in each jaw; the grinding-teeth in an adult squirrel may 



be eighteen in 

 number, -- four 

 on either side 

 of the lower jaw 

 and five on either 

 side of the upper 

 jaw. Of the 

 grinding-teeth 

 the last three 

 on either side 

 in both jaws are 

 termed molars, 



the others pre- 

 FIG. 211. Skull of Squirrel , ~ f 



molars. One 01 



the premolars in the upper jaw is very likely to be minute, or 

 even missing, having been shed in early life. The complete 

 dentition can be expressed in a simple formula, using letters 

 to stand for the names of the teeth, and placing the lower 

 jaw below the line, thus : 



.2 4 6 12 . 



i -i pm '-a m '6 SB i6 s 



or, because each side of each jaw is similar to the other, 

 .1 236 



A tooth contains & pulp-cavity (Fig. 212, 4) supplied with 

 blood-vessels and nerves, and surrounded by a mass of firmer 

 tissue, or dentine (Fig. 212, 3), which makes up the bulk of the 

 tooth. The dentine is usually covered, where the tooth pro- 

 jects from the gum, with a very hard, smooth substance called 



