ANCESTRY OF THE MAMMALIA 61 



heel of the upper molar. Whereas the talonid may have even 

 three cusps the talon never possesses more than one. These 

 facts are mentioned at this stage merely to indicate in what 

 manner the upper molars increase in complexity. No hypocone 

 is exhibited by the molars of Jurassic Mammals. If one ex- 

 amine the postcanine teeth of modern or even of Tertiary Mam- 

 mals the striking similarity of the last premolar to the first 

 molar suggests that the premolars represent stages in the 

 development of the molars. This resemblance gave rise to the 

 Premolar Analogy Theory the basic principle of which is 

 that the premolars exhibit simpler stages in the formation of 

 the molars. Premolars were distinctly marked off from molars 

 even in Triassic times (Diademodon, Protodonta). The simi- 

 larity which they exhibit to the molars in Tertiary and in 

 recent Mammals has been explained on this account as an 

 example of convergent evolution. But we have already noted 

 that there has been no migration or rotation of cusps as was 

 formerly thought to have occurred in the ancestral history of 

 Mammals and, further, that the para cone of the upper molar 

 and not the protocone is the real homologue of the protoconid 

 of the lower tooth. There is therefore no reason for retaining 

 the Scott-Osborn view that the outer cusp of the upper pre- 

 molar is the homologue of the inner cusp (protocone) of the 

 molar. Inasmuch as the premolars in many Mammals increase 

 in complexity from before backward up to the fourth which 

 resembles but is less complex and more primitive than the 

 first molar, it now appears likely that as exemplified by some 

 mammalian forms the premolars do represent stages in the 

 history of the molars. In the Horse the premolars have taken 

 on a completely molariform appearance ; in the Dog they have 

 shown little tendency to become molariform at all. These are 

 not examples from which information can be gleaned regard- 

 ing the relationship of premolar patterns to stages in molar 

 formation. The dentition of the Tapir illustrates the case much 

 better for in the premolar series of this animal each succes- 

 sive tooth is more molariform until the last is barel}^ distin- 



