1912] Miller: Pacific Coast Avian Palaeontology 107 
The exact time-relations between the several faunas is not 
determinable, and the overlap of one column upon another is 
purposely indefinite. The Equus fauna is considered in part 
older than the Megalonyx fauna and this in turn than the 
Ovibos. 
It must be stated also that the study of mammalian remains 
from Rancho La Brea, from the eaves of California, and from 
Fossil Lake, Oregon, is still being actively pursued and the list 
of species revised. Any statement of time-relations must be 
considered as purely tentative. Few investigators have had so 
wide and so comprehensive an acquaintance with the mammalian 
palaeontology of North America as has Professor Osborn; hence 
it is considered in this connection that his chronological arrange- 
ment of the various mammal-bearing horizons represent the 
truth as nearly as we have yet arrived at it. 
It will be noted that the Fossil Lake horizon is placed by 
him midway in the tabulation of the Equus Zone fauna while 
Rancho La Brea and the caves occupy the middle and upper 
parts of the Megalonyx Zone. Thus Fossil Lake is to be con- 
sidered as the earliest Pleistocene horizon on the coast produe- 
tive of avian remains. 
If we apply the criterion of percentage of extinct forms, 
the evidence furnished by the avian remains would indicate 
a different time-relation than that suggested by Professor Os- 
born. The various horizons here discussed show the following 
sequence when arranged according to the percentage of Recent 
species of birds recorded fossil: 
Riancho! Wis s Breas cccescccecs cnceerccee-s> 60% still living 
MOSS Wak Gy osu reeees-b-cccegencevs .......66% still living 
Potter Creek Cave .u..............cs2000--0: 68% still living 
SSRI Wn RV Otte cee renee otc eternsasreeccas 72% still living 
ETEUWVET CAV GC  feecexcscceccsrevcsecennceonse 79% still living 
The application of this principle in the case of fossil birds 
seems, however, less accurate than in the case of mammals when 
we consider the migratory nature of many bird species. The 
Fossil Lake fauna aceording to this basis of estimate would 
appear to be younger than that of Rancho La Brea. A glance 
at the list of species from Fossil Lake shows, however, the large 
