138 DELOS AND RAEPH ARNOLD 
coast since the beginning of Pliocene times. The San Pablo 
(middle Neocene) and Merced faunas contain an abundance of 
such genera as Arca, Dosinia, Pinna and other southern forms 
which indicate that southern or warm conditions prevailed dur- 
ing the period of deposition of those two formations. Follow- 
ing the Merced (or, more strictly speaking, the middle of the 
Merced) comes the San Diego formation with its fauna of such 
northern species as Pecten caurinus, Panomya ampla, Purpura 
crispata, several species of Zvophon and many other species indig- 
enous to northern waters, which shows that northern or boreal 
conditions prevailed during the later Pliocene and early Pleisto- 
cene times. The transition from the boreal conditions of the 
late Pliocene to the semitropical conditions of the upper San 
Pedro series (typical Pleistocene) is indicated by the tran- 
sition fauna of the lower San Pedro series. The upper San 
Pedro fauna contains so many southern species that it is safe to 
assume that conditions more nearly approximating those of the 
coast of Mexico at the present time, prevailed along the coast of 
southern California during that part of the Pleistocene in which 
the upper San Pedro beds were deposited. Since these latter 
strata were deposited a slight change has taken place, and the 
conditions now prevailing were inaugurated. 
DELOs ARNOLD, 
RALPH ARNOLD. 
STANFORD UNIVERSITY, 
California. 
