E 1887] | The Perissodactyla, 997 
latter family. The latter only differs from the former in the 
increased complexity of the premolars. The passage to the 
horse line is suggested by the resemblance of Pliolophus 
oP E 
SE 
a ee =, 
Fic, 9. Heptodon ventorum Cope, superior molars of left side, natural size, 
Original; from Wind River beds of Wyoming. From “ Report U. S. Geol. Surv. 
Ters.” F, V, Hayden, vol. iii. 
(= Orohippus Marsh) to Anchitherium of the family of the 
‘aleotheriide. The transition is seen in some genera of the 
intermediate family of the Chalicotheriide, beginning with 
Ectocium (Cope). 
Fic. 10. Hyrachyus agrarius Leidy, superior molar teeth, natural mee oe 
Bridger bed of Wyoming. From Leidy, “ Report U. S, Geol. Surv. Terrs.,” F. V. 
vol. i. 
Three species of Systemodon are known, all from the Wasatch 
E The largest, S. tapirinus Cope, was equal to a ia 
ay A dozen species of Hyracotherium are known from - 
“ocene of Europe and North America, which range in size 
