428 E. S. GOODRICH. 



posterior surface. So averse, however, is he to damaging 

 Osborn's theory, that he is actually driven to the conclusion 

 that similar cusps in precisely the same situations in the pre- 

 molars and the molars are not homologous, but of entirely 

 different origin P 



Another argument — perhaps the strongest of all — against 

 the assumption that the Triconodont tooth represents a stage 

 in the evolution of the Tritubercular sectorial type is afforded 

 by the consideration of the occurrence of the latter in the 

 various orders of Mammalia, and the probable phylogeny of 

 the Ditrematous mammals. That the Tritubercular sectorial 

 type was that of the lower molars of the ancestors of all the 

 placental mammals can scarcely be doubted in the face of the 

 mass of evidence collected by Cope, Osborn, Schlosser, and 

 others. All the various forms of molars met with amongst the 

 different orders, as adaptations to special food or methods of 

 feeding, can be referred back to this primitive type.^ Now the 

 Tritubercular sectorial molar occurs also amongst the Marsu- 

 pials; we must, therefore, conclude that the common ancestor 

 of both Placeutals and Marsupials possessed teeth of this type; 

 for we cannot assume without evidence that this complicated 

 yet definite pattern arose independently in the two cases : the 

 resemblance between the arrangement of the cusps in these 

 teeth in the two groups is not vague and general, but definite 

 and detailed. 



We have, on the other hand, conclusive evidence that the 

 Triconodont type of tooth has independently arisen in at least 

 two widely separated groups, namely, the Phocidse and Carni- 

 vora, and the carnivorous Marsupials (Phascolotherium, Thy- 

 lacinus, &c.) ; in the former certainly, and in the latter most 

 probably, by the reduction of the cusps of the primitively 

 Tritubercular sectorial molar. 



1 Speaking of the Insectivora, Scott naively remarks that " in this group, 

 strange to say, the oldest member yet discovered exhibits the most com- 

 plicated premolar structure " (28). 



^ Dr. Forsyth Major (10) does not favour this view. However, even in 

 the teeth he figures, traces of the Tritubercular sectorial plan may be easily 

 detected. 



