DALL: MOLLUSCA AND BRACHIOPODA. Pill 
flourished at a time when the Antarctic region had a milder climate, 
it ig not practicable yet to determine. It is, however, certain that 
there is a considerable austral element in the existing faunas, both 
littoral and benthal. 
The contributions of boreal seas to the fauna of the eastern Pacific are 
less easy to determine and less numerous in species under any hypothe- 
sis, since when a species extends from one end of the earth to the other 
it is difficult to determine from which end it originally started. 
Lastly, there is an element, of which the extent is still uncertain, due 
to migration from the Antillean region and adjacent shores of the conti- 
nent at a time when the passage between the two seas in the region of 
Panama or elsewhere was not obstructed. Probably a more or less con- 
stant migration from shoal water to deeper is going on now, and has 
always gone on since littoral faunas existed, when barriers of land or of 
temperature have not intervened. 
There is much in the distribution of the present marine invertebrate 
fauna of South America, east and west, to support the view so strongly 
urged by von Ihering and others that a barrier existed between the north 
and south Atlantic basins during late geologic time, making it difficult 
for the South European animals to reach the South Atlantic unless by 
the roundabout way of the Red Sea and Indian Ocean. In fact, without 
some such barrier it seems impossible to account for some of the facts of 
the present distribution of marine animals. 
But that the influx from the Antillean region of its essentially Medi- 
terranean faunal elements into the Pacific by way of the gaps between 
North and South America was so small as it appears to be seems to 
need explanation. It may possibly be accounted for by the hypothesis 
that the immigrants from the Atlantic found the ground already well 
occupied by a Pacific fauna ; but however we attempt to explain it, the 
fact remains, that the Antillean forms on the Pacific coast are almost 
exclusively confined to shoal water, while the Pacific coast types like 
Strombina and Fasciolina, though found abundantly in the Tertiaries of 
the Texas coast and the West Indies, have survived the Pleistocene 
only on the shores of the Pacific. 
In conclusion I have to thank the authorities of the U. S. National 
Museum and Smithsonian Institution, the Director of the U. S. Gevlog- 
ical Survey, and the Librarian of Congress, for facilities offered and 
utilized in the preparation of this report. To Dr. H. A. Pilsbry of the 
Academy of National Sciences, Philadelphia, I am also indebted for 
advice and information received. The late Dr. J. C. McConnell prepared 
