dall: mollusca and brachiopoda. 211 



flourished at a time when the Antarctic region had a milder climate, 

 it is 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 laud 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 lato 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 uf 

 the present distribution of marine animals. 



But that the influx from the Antillean region of its essentially ^ledi- 

 terranean faunal elements into the Pacific by way of the gaps between 

 North and Si)uth 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 \\v\\ 

 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 I". S. Geuloi:- 

 ical Survey, and the Librarian of Congress, for facilities oftered and 

 utilized in the preparation of this report. To Dr. II. A. Pilsbry of the 

 Academy of National Sciences, Piiiladelphia, I am also indebted for 

 advice and information received. The late Dr. J. C. McConnell prepared 



