ZOOLOGICAL POSITION AND STRUCTURE 23 



(thereby reducing the number of cheek-teeth from 

 six to five), it may be noted that the third lower 

 tooth in the earlier stage consists of three distinct 

 lobes, whereas in the later condition the corres- 

 ponding tooth is composed of one main lobe, with a 

 supplemental lobule on the posterior side. 



As regards the front teeth of the lower jaw, the 

 first or innermost pair of milk-incisors are shed 

 during the second year and replaced by the per- 

 manent ones, easily recognised by their much 

 broader crowns : it was when in this condition that 

 the sheep was described by classical Roman authors 

 as bidens, that is to say, two-toothed. It will be 

 noticed that at this stage the first pair of permanent 

 lower incisors and the first and second pairs of 

 permanent molars are in place simultaneously with 

 the second and third pairs of deciduous lower 

 incisors, the deciduous lower canine, and the 

 three pairs of deciduous or milk molars of both 

 jaws. 



Towards the close of the second year the 

 third or last pair of permanent molars cuts the 

 gums both above and below ; while at the 

 same time the deciduous milk-molars begin to be 

 replaced by the permanent premolars, falling at 

 short but irregular intervals, and in such a manner 

 that the last of the series may be shed a week or so 

 earlier than the two in front of it. This replace- 

 ment of the cheek-teeth is generally, if not invari- 



