9 8 



GIDLEY 



pressed V-shaped cusp supported by a single fang, centrally 

 placed, and exposed on its inner side for the greater part of its 

 length, the maxillary bone apparently not yet having formed 

 a completed socket, or alveolus, for its reception. Thus the 

 whole construction of the inner cusp, which is highly sugges- 

 tive of a heel development, differs materially from the central 

 cone of Triconodon. 



A 



O O O 



B 



°- = — ° °^y = ° ° = o = ° 



c 



0=0=0 0=0-0 



D 



VAVAV 



E 



F 



J 



Fig. 11. Phyletic History of the Cusps of the Ungulate Molars. A, Reptilian 

 Stage, Haplodont, Permian. B, Protodont Stage {Dromotherium) , Triassic. 

 C, Triconodont Stage {Amphilestes). D, Tritubercular Stage (Spalacothe- 

 rium). E, Tritubercular-tuberculo Sectorial, Lower Jurassic. F, The same, in 

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

 Lower Eocene. /, Sexitubercular-sexitubercular, Puerco. J, Sexitubercular- 

 quadritubercular, Wahsatch. (After Osborn.) 



Considering the outer portion of the Diyolestes molar as 

 homologous to the three cones and two fangs of Triconodon, 



