THE TEETH OF PROBOSCIDEA. 



357 



has in them given place to a succession from behind, the 

 older teeth being pushed out forwards. Had the elephant 

 always been as isolated a form as it now appears to be, it 

 would have been very uncertain how its six molars should 



bo classified. But it happens that proboscideans formerly 

 existed in which this pecular succession from behind was to 

 be found, at the same time that the ordinary vertical suc- 

 cession was not quite lost, and amongst these creatures (the 

 mastodons) we are able to say with certainty which of the 

 teeth are milk molars, which are pre-molars, and which are 

 true molars. And as the mastodons pass by insensible gra- 

 dations into the elephants, so that the line of demarcation 

 between the two genera is an arbitrary one, we can tell 

 which of the mastodon's teeth correspond to each one of the 

 six molars of the elephant. 



Mastodon. In the later tertiary periods this genus, ap- 

 proximating in its dental and other characters to the true 



(') Isolated plate (= exaggerated cusp) of an Elephant's tooth, prior to 

 its coalescence with neighbouring plates ; at the top are seen its terminal 

 mammillated processes, one of which has been cut off to show the central 

 area of dentine, surrounded by enamel ; at the base would be the open 

 pulp cavity, not shown in the figure. 



