208 EVOLUTION OF MAMMALIAN MOLAR TEETH 



later Multituberculates which are the only known group of polybunous 

 mammals in the Mesozoic period, and all of these have sharply defined 

 and modeled cusps, elongate crowns, parallel grinding series, enlarged 

 chisel like incisors, correlated with chiefly horizontal jaw motions and 

 gnawing habits conditions far too specialized to have given rise to 

 any of the later fewer-cusped or tritubercular types. (3) Embryological 

 evidence is against the polybuny theory, there being no trace of previous 

 polybuny, but on the contrary quite generally in the true molars a 

 triangular disposition of the cusps first developed. (4) According to 

 the polybuny theory one cusp is as old as another, but if either the 

 ' tritubercular ' or the ' embryological ' or the ' premolar-analogy ' theories 

 be correct one of the cusps is older than the others. 



II. That the Cope-Osborn Theory of the Origin of the 

 Superior Molars is Incorrect. 



This concerns the superior molars only. Strong evidence is brought 

 forward from embryogeny, from the analogy of premolars, from 

 palaeontology, that in the superior molars the protoconc (of Osborn) is 

 not homologous with the primitive or original reptilian cusp of the 

 crown. 



In previous pages of this volume we have stated and discussed the 

 evidence against the Cope-Osborn theory of the origin of the upper 

 molars. 



Let us now review this evidence. 



1. CUSP HOMOLOGIES FOUNDED ON EMBRYOGENY. 



The most thorough presentation of the development of tooth cusps in 

 its bearings upon the homologies of the upper cusps is that by M. F. 

 Woodward, 1 published in 1896. 



In course of this important article he refers to the deficiency of 

 the palaeontological evidence among trituberculates, so far as the upper 

 teeth are concerned, to establish the homology of the upper and lower 

 protocones beyond question, concluding : " We consequently have no 

 palseontological evidence to support the assumption that a tritubercular 

 stage is passed through by the mammalian upper molar in its evolution 

 from a protodont or possibly a triconodont tooth. ... If the triconodont 

 tooth be a stage in the evolution of the mammalian molar, then I 

 should believe that the anterior cone disappeared, the main cone 

 becoming enlarged as the paracone and the posterior as the 

 metacone. ... At this stage the upper teeth overhang and bite out- 



1 "Contributions to the Study of Mammalian Dentition, Pt. II., On the Teeth of certain 

 Insectivora," Proc. Zool. Soc. Land., 1896, pp. 557-594. 



