THE LAND GASTROPODA IN THE NORTH ATLANTIC REGION 161 



Table 2, as well as Fig. 2, particularly well demonstrates the importance of 

 the interchange in a southeastern direction. A long series of Tertiary European 

 genera, or relatively closely related forms, today are met in South and even 

 East Asia. To these is joined a more limited element, which occurs both in 

 South Asia and in Africa, mainly in the eastern parts. From the point of 

 view of principles it is of secondary importance whether the genera are 

 European emigrants, or if they have spread from, for example, an Asiatic 

 center of origin to Europe, and later become extinct there. 



An exclusive African affinity, wliich does not seem possible to connect 

 to the South Asiatic route of dispersal, is shown by very few genera only. 

 Except for three Paleocene genera which, indeed, are rather difficult to 

 delimit from the previously regarded Archeotropical element, only the recent 

 genus Lauria belongs to this group. However, as this genus is comprised of 

 small forms well adapted for passive dispersal, for instance, by birds, its 

 distribution is not very significant. In fact, also some Paleo- or Holarctic 

 genera, with similar qualifications for passive dispersal, have adjoining 

 areas in Africa, mainly on high mountains. For one genus, Truncafellina, 

 the African distribution is so extensive that an African origin is not 

 excluded. 



These conditions in terrestrial Gastropoda are quite compatible with 

 modern conceptions regarding the pattern of evolution of tropical African 

 biota (cf. Moreau, 1952), dominated by the long and efficient isolation. 



However, in recent time the European land gastropod fauna is as completely 

 isolated from the South and East Asiatic one as it has been from the African 

 one throughout the Tertiary. The part of the Mediterranean fauna, which has 

 its center in northwestern Africa, has scarcely more points in common with 

 the tropical African one; its affinities are almost exclusively European. 



For the Holarctic genera, which will be mentioned later, the dispersal 

 over the Euro-Asiatic territory has been of fundamental importance. These too 

 can be seen as an example of interchange between Europe and Asia, though in 

 a more northerly latitude, and less influenced by climatical and geographical 

 barriers during the Tertiary. 



Disregarding the possibility that an interchange of Holarctic taxa has also 

 taken place in the North Atlantic area (though, for several reasons, to a 

 limited degree only), there are no traces whatever of any contributions to the 

 European land gastropod fauna from North America. Likewise, in the 

 opposite direction, there are no incontrovertible examples of dispersal, 

 except the doubtful, and in every case late, instances of Cepaea and Umax 

 which will be attended to further on. 



However, a careful examination makes evident that for some taxa, which 

 are common to the warmer parts of Europe and America, the possibility of a 

 trans-Atlantic dispersal must be considered. But the nature of this dispersal 

 seems enigmatic in the light of the facts that are known at present. 



