360 



MECHAi^IOAL EVOLUTION". 



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 in- 

 ternal crests with the anterior external. Its primitive form is 

 seen in Didelphys, Pelycodus, Pantolambda, and the Amblypoda 

 generally ; in Centetes and Talpa ; and in its sectorial form, in 

 Stypolophus and Oxyaena, etc. 



Fig. 64. — StypolopTma whiticB Cope, skull two thirds natural size. A Creodont 

 from the Wasatch Eocene, displaying well the tritubercular superior, and the tubercu- 

 lar-sectorial inferior molars. 



The mechanical action of such teeth is as follows : Of course, 

 it results from the form of the superior molars that the spaces be- 

 tween them are wedge-shaped, the apex external, the base open- 



