64 University of California Publications in Geology |Vou.7 
basis in the determination of mammals than is the case with 
birds. Recognizing this principle, the author of this paper has 
proceeded with perhaps more than necessary caution in the an- 
nouncement of new species, preferring to err on the part of 
conservatism rather than to confuse the literature of the subject 
by making assertions which must later be modified ; and there are 
in the collections studied many specimens regarding which fur- 
ther knowledge is considered necessary befere problems upon 
which they may throw light can be attacked in more than a 
speculative way. 
REVIEW OF THE LITERATURE 
Since the epoch-making discoveries by Marsh which added 
so materially to our conception of the ancestry of birds, con- 
tributions to knowledge in the field of avian palaeontology have 
been few as compared with the rapid enlargement of our under- 
standing of the other vertebrate groups. Bird remains on the 
Pacific Coast are mainly from Pleistocene strata; thus there is 
eliminated the probability of shedding much new light upon the 
ancestry of certain groups in which our interest is so acutely 
focused, for example the Stereornithes. Discoveries recently 
made have contributed to science chiefly in two ways, first in 
giving us an appreciation of the relative antiquity of the main 
groups into which birds are divided; and, second, in adding to 
our knowledge of the geographical distribution of these groups. 
The consideration of geographical distribution is but begun when 
we record the range of the Recent species. Determination of 
the factors which have led to such distribution, if we aspire to 
something better than mere speculation, must look to the record 
of previous conditions as brought to light through palaeontol- 
ogiceal inquiry. 
The fossil-bearing rocks of the Pacifie Coast of North 
America, while rich in the remains of mammals and reptiles, 
have until recently yielded but little information concerning the 
avian group. 
In 1878, Cope! described three new species of birds from 
the Equus Beds of Oregon. All three species belong to genera 
1 Cope, E. D., Bull. U. 8S. G. 8., Terr., iv, No. 2, May 3, 1878. 
