2 go PRIMARY FACTORS OF ORGANIC EVOLUTION. 



that without exception, every line of Mammalia com- 

 menced with types with an astragalus which is flat in 

 the transverse direction, or without median groove. 



n 



IN W 1 



w r 



Fig. 69. Fig. 70. 



Fig. 69. Hind foot of primitive cameloid Poebrotherium labiatum Cope, 

 showing grooved astragalus and first toe-bones without keel in front at lower 

 end. (From Colorado.) 



Fig. 70.- Hind foot of three-toed horse (Prothippus sejunctus Cope; (from 

 Colorado), showing grooved astragalus, and trace of keel on front of lower 

 end of first bone of middle toe. 



