168 



EVOLUTION OF MAMMALIAN MOLAR TEETH 



m.l 



Batliyopsis. 



Coryphodon. 



m 3 



m .3 



Pantolainbda. 

 Firs. 145. Evolution of the lower molars of Arnblypoda. Internal view. 



A 



p. 2 p. 3 p. 4 m ,/ m2 



m, 3 



FIG. 140. Lower teeth ( x \} and mandible ( x %) of Bi'tltyopsis tissidens, an Amblypod from the 

 Wind River Formation, Stage II., Lower Eocene. In the structure of its lower molars this animal 

 is more like Uintathcriwn than Coryphodon. (Of. Fig. 144.) 



-CONDYLAKTHKA. 



These Ungulates are contemporary with the Amblypoda, but 

 unlike them their molar teeth evolve from trituberculy into quadri- 

 tuberculy. 



The proof of the tritubercular ancestry of these oldest and most 

 generalized hoofed animals is furnished by the oldest form Protogono- 

 don (Fig. 148) and by the diminutive predecessor of Phenacodus, the 

 species Euprotogonia minor (Fig, 151); in this species the crown 

 is still triangular, but the hypocone is seen developing on the second 



