290 PRIMARY FACTORS OF ORGANIC EVOIUTION. 



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. 6g. Fig. 70. 



Fig. 6g. — Hind foot of primitive cameloid Poebrotheriunt labiatutn Cope, 

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

 end. (From Colorado.) 



Fig. 70. — Hind foot of three-toed horse {Prot hip/>us sej'uncius Co'pe) (from 

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

 end of first bone of middle toe. 



L 



