32 THE HORSE AND ITS RELATIVES 



mark. Moreover, when this false mark makes its 

 appearance, the section of the crown of the incisor 

 has become much more triangular than in the early 

 stages of wear ; and in extreme old age, when 

 the incisors are worn down to their very roots, 

 these teeth become very narrow in the transverse 

 direction, whereas in their earlier ages this 

 diameter was considerably larger than the opposite 

 one. 



From the foregoing description, it will be evi- 

 dent that to the definition of the horse family given 

 above may be added an additional characteristic, 

 namely the presence in an early condition of wear, 

 of a "mark," or pit, in the crowns of the incisor 

 teeth. As regards the object of these pits, their 

 function is probably to increase the grinding surface 

 of the dentition as a whole during the period when 

 the cheek-teeth have not attained their full develop- 

 ment ; the number of functional teeth of the latter 

 series in young colts being only three pairs in each 

 jaw, while at a later stage, when some at least of 

 the permanent incisors have come into use, the 

 hinder teeth of the cheek-series are not fully 

 developed. 



Before leaving this part of the subject, it may 

 be mentioned as a remarkable circumstance that, 

 in addition to the existing members of the horse 

 family and some of their extinct forerunners, the 

 only mammal which shows a pit in its incisors is 



