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. 



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. 



