1884.] Geology and Paleontology. 281 
of the best marked types of Mammalia the world has seen. The 
history of this type involves more or less the history of the life 
of the North American continent during the Miocene epoch of 
Tertiary time. It moreover involves the laws which regulate the 
vital success of all types of life, and which express the causes of 
multiplication, of energy, of weakness and of sterility. 
Two lines of the family, the Oreodontinæ and the Agriochcer- 
ina, come to light simultaneously in geological time, the White 
River epoch, or the Oligocene. The latter is a higher type than 
the former, in its more complex fourth premolars, while it is infe- 
rior in the non-closure of the orbits posteriorly. It may then be 
regarded as a parallel line. It has but two generic types, while 
the Oreodontinz present us with seven. So far as yet known, the 
Agriochcerine did not continue as long as the Oreodontinz, as 
will be shown in tabular form below. 
In the progressive modifications of the Oreodontine series, the 
first step was the inflation of the otic bulla (genus Eucrotaphus). 
€ species may be thus arranged in accordance with their dis- 
tribution in time: 
rite River epoch——Oreodon gracilis; O. affinis ; O. culbert- 
ae Eucrotaphus jacksoni; E. major. Agriochcerus antiquus ; 
Major ; A. latifrons. 
oo epoch—Eucrotaphus jacksoni; E. major. Meryco- 
““tus superbus; M. leidyi; M. chelydra, sp. nov.; M. macro- 
rips Sp. nov. Agriochcerus guyotianus; A. trifrons, sp. nOV.; 
~fyderanus. Coloreodon macrocephalus. 
North Fork of ¥ohn Day River epoch—Eucrotaphus trigono- 
