GEOLOGIC HORIZONS AND LIFE ZONES. 



15 



Foet Logan, Montana. 

 Type of Parahippus taxus Douglass, 190S.2. 



Incertw sedis, Montana. 

 Type of Parahippus agrestis Leidy, 1S73.2. 



Ticholeptus-Merychippus Zone, Middle Miocene. Successive Helvetian, Sarmatian, Tortonian Stages of Europe. 



This zone is characterized by the continuation, in Montana, of the oreodont Ticholeptus, by the rare survival of Para- 

 hippus, by the first appearance of a pio-Merychippus (P. brcvidens of the Mascall), and by the first appearance of 



Fig. 9. Outline map of the chief Tertiary mammal localities in the United States west 



of the Wasatch range. 



CM Cedar Mountain beds, E Esmeralda, T Truckee, MK McKnight Miocene, V-T Virgin Valley and Thousand 

 Creek beds, Rt Rattlesnake beds and Mascall, Rc Ricardo beds, B Barstow beds. After Merriam, 1916, PI. 8. 



Hypohippus and Archwohippus. The name " Ticholeptus beds," applied by Cope, is not distinctive and may be replaced 

 by "Ticholeptus-Merychippus zone," owing to the continuation in Montana of the oreodont Ticholeptus. 



1. Horizon E, central plains, Hayden and Leidy, 1S69. 



2. Mascall formation (overlying John Day formation), Ore., Cope, 1879, Merriam, 1901. 



3. Ticholeptus beds, Montana, Cope, 1879. 



