MOLAR TEETH OF THE MAMMALIA EDUOABILIA. 245 



and of the remaining sixty-five, all are triangular, excepting those 

 of three species of Periptychus, and three of Oonoryctes, which 

 have a small supplementary lobe on each side of the median prin- 

 cipal inner tubercle. 



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 Marsupi- 

 alia ; in the Creodonta, and the tubercular molars of such Carnivo- 

 ra as possess them (excepting the i^lantigrades). In the Ungu- 

 lates its persistence is to be found in the molars of the Corypho- 

 dontidse of the AYasatch, 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 {Anisonchus), and in 

 the sectorials of the Procyonidae. 



2. The Mandibular Teeth. 



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 I have called the tubercular 

 sectorial, and is very variable as to the degree of development of 

 the sectorial cutting edge. The anterior triangle is formed by the 

 connection by angle or crest, of the median and anterior internal 

 crests with the anterior external. Its primitive form is seen in 

 Didelphys, Pelycodus, Pantolambda and the Amblypoda gener- 

 ally ; in Centetes and Talpa ; and in its sectorial form, in Stypo- 

 lophus and Oxyaena, etc. 



The * tubercular molar of some ViverridcB, and among the ex- 

 tinct forms especially the Didymictis protenus, Cope, present a 

 similar structure to that just described. This furnishes a ready 

 explanation of the tooth immediately in advance, which is the 

 primitive form of sectorial tooth characteristic of primitive Car- 

 nivora. The three anterior tubercles are largely developed, stand- 



* The remainder of this section (2) is taken from the writer's " Synopsis of the 

 Vertebrata of the Eocene of New Mexico," p. 800. Publication of the U. S. G. G. 

 Survey, W. 100th Meridian, 1875. (Ed. 1886.) 



