XIIL 



ON THE TRITUBERCULATE TYPE OF MOLAR TOOTH 



m THE MAMMALIA. 



It is now apparent that the type of superior molar tooth which 

 predominated during the Puerco epoch was triangular or trituber- 

 cular ; that is, with two external and one internal tubercles.* 

 Thus, of sixty-seven species of placental Mammalia of which the 

 superior molars are known, all but four have three tubercles of the 

 crown, and of the remaining sixty-five, all are triangular, except- 

 ing those of three species of Periptychus, and three allied forms, 

 which have a small supplementary lobe on each side of the median 

 principal inner tubercle, f 



This fact is important as indicating the mode of development 

 of the various types of superior molar teeth, on which we have not 

 heretofore had clear light. In the first place, this type of molar 

 exists to-day only in the insectivorous and carnivorous Marsupialia ; 

 in the Creodonta, and the tubercular molars of such Carnivora as 

 possess them (excepting the plantigrades). In the L^ngulates its 

 persistence is to be found in the molars of the Coryphodontidae of 

 the Wasatch, and Dinocerata of the Bridger Eocenes. In later 

 epochs it is occasionally seen only in the last superior molar. 



It is also evident that the quadritubercular molar is derived 

 from the tritubercular by the addition of a lobe of the inner part 

 of a cingulum of the posterior base of the crown. Transitional 

 states are seen in some of the Periptychidae {Anisonchiis), and in 

 the sectorials of the Procyonidae. 



The tritubercular or triangular superior molar is associated 

 with a corresponding form of the anterior part of the inferior 

 molar. This kind of inferior molar X I have called the tubercular 



* See " American Naturalist," April, 1883, p. 407. 



f This type is tlierefore only an extension of the tritubercular. (Ed. 18S6.) 

 X See Report G. M. Wheeler, D. Chief of Engineers on Explor. Surv. W. 100th 

 Mer., vol. iv, pt. ii ; on the Creodonta, 



