H. F. Osbovn — Mammalia in North America. 



449 



porting three cusps, as found in the Cretaceous. Consider the 

 extreme antiquity of the three homologous cusps borne upon 

 the back part of the human molar. This addition gave the 

 opposed molars two shears and one crusher, and was so per- 

 fectly adapted to the needs of Lemurs and many Insectivores 

 and Carnivores, in short, of most clawed animals, that they 

 stopped at this point. Not so with the Herbivora, which 

 required more extensive crushing surfaces. The upper molars, 

 which had remained triangular through the Cretaceous and 

 into the basal Eocene, began to develop a little talon, like that 



o o o 



B 



•-#-• •~^» «-#^» 

 <K>° <y= Q =o 0=0° 



o==0=o 0=0=0 



" VAVAV 



F 



J 



Phyletic History of the Cusps of the Ungulate Molars. 

 Upper molars in heavy lines, loioer in light lines. 



A, Reptilian Stage. Haplodont. Permian. B. Protodotit Stage (Dromotherium), 

 Triassic. C. Triconodont Stage (Amphi testes). D. Tritubercular Stage ( Spala- 

 cotheriuin. E. Tritubercular-ti.berculo Sectorial {Amphitherium).* F, The same, 

 in Upper Jurassic. G, The same, in Upper Cretaceous. H, The same, Puerco. 

 Lower Kocene. I. Soxitubercular-sexitubercular, Puerco. J. Sexitubercular- 

 quadritubercular. Wahsatch. 



* Since this drawing was published I have learned that the talon of the lower 

 molar of Amphitherium is considerably larger than here represented. 



