702 
PALEONTOLOGY: W. H. DALL 
Pachydesma stuUorum Mawe. Southernmost mainland locality lati- 
tude 21° 36' N., San Bias, Mexico. 
Protothaca staminea Conrad. Southernmost mainland locality lati- 
tude 24° 40' N., Magdalena Bay, Lower California. 
Pholadidea sagitta Stearns. Southernmost mainland locality lati- 
tude 32° 24' N., San Diego Bay, Cahfornia. 
There are also several gastropods, but on none of the islands cited 
has an exhaustive collection been made, so that a complete list is still 
a desideratum. 
Mr. W. H. Ochsner, naturahst of the Galapagos Expedition of the 
California Academy of Sciences was the first to discover and collect 
from fossiliferous sedimentary strata on those islands. 
In the case of the recent fauna of Socorro Island, we have at least four 
characteristically Calif ornian species surviving on the island from 200 
to 844 geographical miles south of their southernmost known extension 
on the mainland. 
As the distances are calculated only from the difference in latitude 
and in geographical miles, while the coast line trends in a southeasterly 
direction, the actual distances are very considerably larger. 
What explanation can be offered of these remarkable anomalies? 
The facts here presented, with others not yet fully worked out, have 
led to the framing of the following hypothesis. 
Suppose the islands and banks here briefly referred to are the remnants 
of a peninsula or chain of islands once existing, parallel to the present 
peninsula of Lower California and connected to the mainland of southern 
California. During the existence of this peninsula, the cold current 
down the coast which now gives rise to fogs as far south as Cerros Island, 
in Pleistocene time, especially during the Glacial Epoch, might have 
carried its normal fauna far to the southward ; this peninsula at the same 
time protecting from the cold current the waters, normally warm, of 
the sea between it and Lower California as well as of the Gulf of Cali- 
fornia, so that the tropical fauna might creep up to the extreme north- 
ern hmit of these enclosed waters. 
With the subsidence of the water barrier most of the northern fauna 
would have perished, but on the undrowned peaks a few of the more 
adaptable of the northern forms might have been able to persist or have 
left their traces in the subsequently slightly elevated Pleistocene sedi- 
ments. 
Our present knowledge of the seabottom west of the inshore deep 
trough previously referred to, is insufficient to add support to this 
hypothesis, but the stupendous changes of elevation on the California 
