294 A MANUAL OF DENTAL ANATOMY. 



The development of additional cusps from up-growths of the 

 cingulum, and the suppression or fusion of pre-existing cusps, 

 may be traced by a comparison of the teeth of allied animals, 

 and thus connecting links are found between patterns at 

 first sight very dissimilar. The order Proboscidea affords, 

 however, so instructive an example of the manner in which 

 an exceedingly complex tooth has been derived from a 

 simple one, that it may be mentioned in this place as an 

 example. 



The tooth of the elephant is so strikingly unlike other 

 teeth that it might at first sight be supposed that it is 

 more essentially different than is really the case. The clue 



to its nature is afforded by the teeth of an extinct Pro- 

 boscidian, the Mastodon. If we take as our starting point 

 the second true molar of one of the Mastodons (Tetralo- 

 phodon) we find its crown to be made up of four strongly 

 pronounced transverse ridges, the summits of which are 

 made up of rounded eminences (whence the name Mastodon, 



(*) Second upper molar of Mastodon (longirostris), from Falconer. 

 About one-eighth natural size. The four transverse ridges, b, c, d, e, are 

 seen to be, to some extent, divided into outer and inner divisions by a 

 longitudinal cleft, much less deep than the transverse indentation. At 

 the front there is a slight elevation of cingulum into a " talon '' (a), and a 

 similar one at the back of the tooth ; by its further elevation additional 

 ridges or cusps would be formed. 



