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displaced during later periods are Carididae, Cerithiidae, 
Fissurellidae and the genera Arca and Mactra (Mulinia). 
This condition indicates the additions that have 
been made to the fauna through migration. In this are 
to be counted, in the first place, those genera that are 
represented in the Chilisn Tertiary, and fail in the 
Argentinian or, asin the caseof Mactra, were represented 
by species of different sections. Besides others, we have 
the genera Purpura, Monoceros, Concholepas and Mulinia 
which were represented inthe Chilian Tertiary but later 
migrated to the Magellan District. - 
A number of species seem also to have migrated 
from more northern latitudes, but Ishall return to this 
point later. Immediately after this there occured also a 
late Tertiary or, what is more probable, a Pleistocene im- 
migration from Antarctic regions which to a large extent 
pme the general character of the fauna. It is not 
possible to derive the entire rich representation of the 
genus Fissurella iv the Magellan District and Chile from 
the single small representative in the Patagonian Ter- 
tiary. Apart from this, Fissurella species are not repre- 
sented in the Tertiary of Patagonia and Chile, and 
species of the genera Acmaea, Patella, Gadinia, Siphonaria, 
Bullia are wholly wanting in the Argentinian and 
Chilian Tertiary. The wide distribution of many of the 
species considered in this connection over the Antarctic 
District to New Zealand (Siphonaria redimiculum Rye.), 
Cape of Good Hope (Patella barbara L,) Kerguelen, New 
Zealand islands etc. (Patella aenea Gm.) points toa rela- 
tively late immigration out of Antarctic regions. One 
species (Siphonaria Lessoni Blvl.) considered in this con- 
nection occurs on the Patagonia-Argentinian coast and 
also on the Pacific coast of South America and, indeed, 
extends here a great deal farther north than on the 
Argentinian coast, following the more northerly position 
of the Isotherme. If a form thus distributed on two sides 
of the continent becomes extinct at its point of origin 
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