THE HEAD AND NECK. 



131 



There is still a limit to the growth of their teeth. 

 After a length is attained which, under normal con- 

 ditions, supplies sufficient grinding-surface to last 

 the lifetime of the animal, a neck and roots are 

 formed, and the tooth is reduced to the condition 



FIG. 20. a, Lower molar of Anchitherium (brachy- 

 dont form) ; &, lower molar of a young horse, with 

 the crown slightly worn and the roots not yet 

 formed (hypsodont form) ; c, the same tooth of an 

 old horse, with the crown almost entirely worn . 

 away and the roots fully formed. 



of that of the brachydont ancestor (see 6 and c, 

 Fig. 20). 



It is perfectly clear that this lengthening of the 

 crown adds greatly to the power of the teeth as or- 

 gans of mastication, and enables the animals in which 

 it has taken place to find their sustenance among 



