62 MAMMALIAN DENTITION 



guishable from the first molar. In the main then the Pre- 

 molar Analogy Theory as presented at the present day is 

 correct. 



The latest presentation of the Theory of Trituberculy as set 

 forth by Gregory, while emphasizing the belief that the para- 

 cone actually represents the summit of the primitive reptilian 

 crown and is therefore homologous with the protoconid, insists 

 that the primitive condition of the molars in both jaws was 

 one of reversed triangles or wedges, that the upper and lower 

 molar crowns were not alike but strikingly dissimilar and that 

 there has been no migration of the cusps which originally ap- 

 peared in the situation where they now exist. According to 

 this " Wedge" Theory the ancestors of the Trituberculates were 

 neither Triconodonts nor Protodonts but some unknown Cyno- 

 dont with transversely widened upper molars. In Tritubercu- 

 lates the upper molars may indeed be called tritubercular 

 whereas the lower teeth have always possessed a heel or talonid 

 in addition to the trigonid and are therefore more properly 

 termed tuberculo-sectorial teeth. We shall follow these same 

 types in the differentiation of the teeth of modern Mammals 

 and we shall note that of the two the lower molar is the more 

 stable in pattern. 



Recent search among fossil animals for a parent type of 

 later Mammals has established the claim that modern Mam- 

 mals originated from Jurassic Trituberculate-like forms but 

 the ancestry of the Trituberculates themselves is obscure. 



In computing the relative age of marsupial and placental 

 Mammals it is interesting to speculate upon the precise relation- 

 ship of the Trituberculata. Trituberculates like Amphitherium, 

 as indeed Triconodonts also, exhibit many characters now 

 best illustrated by the Marsupials. But many of these fea- 

 tures may be and probably are specializations and indicate 

 merely that the animals are not directly in the mammalian 

 ancestral line. The same features may be found in the more 

 specialized Pla centals. Varying inflection of the mandibular 

 angle for example is exhibited in slight degree by several 



