1 58 HENRIK W. WALDEN 



complicated in Europe; this is reflected in the markedly richer taxonomical 

 differentiation. In recent times, 229 genera are indigenous to Europe. For the 

 number of species, however, no actual, reliable figures exist, but they may be 

 estimated as more than double those of North America. On an average, 

 however, the geographical splitting of taxa takes place on a lower taxonomical 

 level than in North America. The major causal background — the highly 

 complicated history of the European mountain upfoldings and seashore 

 dislocations — has led to isolation processes which were richly varied, but 

 only moderately persistent. However, the details are of infra-European 

 interest only, and consequently they can be largely ignored here. 



The principal traits in the development of the European land Gastropoda 

 is demonstrated by Fig. 2. 



As already pointed out in the preceding paragraph, the land gastropod 

 fauna which appears in Europe in the Late Mesozoic had no points in 

 common with the contemporaneous North American fauna. But, in addition, 

 it has very little in common with the recent European. Except for some 

 unimportant, very specialized remnants, the Mesozoic genera today are 

 quite absent from Europe. In 88.5 per cent (24 of 27 genera), the connections 

 were Tropical, either with the recent South Asiatic and African gastropod 

 fauna, or, for the greater part, representing an Archeotropical element which 

 is difficult to evaluate taxonomically. 



Some authors have made the fascinating, though very hypothetical sugges- 

 tion that this Archeotropical element partly represents an old "Gondwana" 

 fauna. 



However, from the Paleocene onwards the recent European genera appear. 

 At first, such genera, which today are Holarctic, form a relatively large 

 fraction. Later, more and more endemic European genera are added. The 

 Archeotropical element rapidly disappears, whereas still in the Eocene that 

 which has connections to South Asia and Africa constitutes nearly 40 per 

 cent of the land gastropod fauna. Then it continuously decreases, a process 

 which no doubt reflects the climatic development during the Tertiary. In this 

 respect the trend entirely parallels that of the marine Mollusca (Davies, 1934), 

 and of the flora (Reid, 1935). 



Up to and including the Miocene nearly half the endemic European 

 element is constituted by subsequently extinct genera. After this period, 

 however, their number rapidly decreases. Also this process probably should 

 be considered against the background of the climatic development. In the 

 Phocene, the European land gastropod fauna has essentially its recent charac- 

 ter; 87 per cent of the genera of this period still live in Europe. From the 

 Pleistocene only recent genera are known, if the very doubtful record of 

 Archaeoxesta in Germany is disregarded. This record may be due to redeposi- 

 tion from Tertiary strata. 



To conclude, the European land gastropod fauna has kept less of its earlier 



