HISTORICAL AND TAXONOMICAL ASPECTS OF 



THE LAND GASTROPODA IN THE NORTH 



ATLANTIC REGION 



Henrik W. Walden 

 Museum of Natural History, Gothenburg, Sweden 



A ZOOGEOGRAPHICAL divisioii with respect to land Gastropoda rather markedly 

 differs from the conventional one, which is based mainly on the distribution 

 of vertebrates (cf. Darlington, 1957). However, more thorough studies of 

 the differences show that an essential causal consistency exists. Somewhat 

 schematically, the terrestrial Gastropoda can be characterized as a conserva- 

 tive group, whose actual distribution and taxonomy to a rather remarkable 

 degree reflect the past. On the contrary, they less obviously react to short- 

 term changes. The rich paleontological evidence offers substantial basis for 

 speculations concerning the faunal development. 



For an understanding of the recent distribution of the gastropod fauna on 

 both sides of the North Atlantic it is necessary to follow its development 

 since the late Mesozoic. At that time a terrestrial gastropod fauna, with 

 obvious points in common with the present one, appears for the first time. 



At this era the gastropod faunas in Europe and North America are already 

 thoroughly distinct from each other, and they have largely remained so till 

 the present time. However, this does not imply a parallel development — on 

 the contrary, very marked differences in the trends of development exist. These 

 differences are of importance to the recent distribution of the land Gastro- 

 poda. 



Owing to the presence of modern, comprehensive literature, it is easier to 

 demonstrate how the gastropod fauna has been formed in North America 

 than in Europe. The European literature in the field no doubt is much richer 

 than the American, but for obvious reasons it is highly diverse from the 

 point of language as well as quality. These circumstances should be kept in 

 mind in the following discussion. 



Taxonomy, distributional and paleontological data mainly follow the 

 fundamental works of Wenz (1961-2), and Wenz and Zilch (1959-60), in 

 addition to which Pilsbry (1939-48), and Henderson (1935), constitute the 

 main sources concerning North American Gastropoda. Certain modifica- 

 tion, especially concerning fossil records, has been undertaken in accordance 



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