A criticism of the Tritubercular Theory of Cope and Osborn. In addition to the tritubercular upper molar tooth, 2 other types are distinguished in primitive Cenozoic and Cretaceous mammals-dilambdodont and zalambdodont. Lower molar patterns are correlated with those of the upper molars. The relationship between the 3 types is discussed in the light of the teeth of Jurassic mammals, with the conclusion that the tritubercular molars are more advanced than the dilambdodont and zalambdodont molars. This is borne out by an analysis of their oo clusal relationships.