168 HENRIK W. WALDEN 



Detailed comment on the diagram might be unnecessary. The extensive 

 negative faunal character of Greenland is conspicuous. Obviously this is the 

 consequence of combined cHmatic and dispersal obstacles. In principle the 

 categories of land Gastropoda show a similar reaction, but most strikingly 

 in the climatically more pretentious endemic groups. 



On the American side the Davis Strait forms a nearly absolute barrier. 

 Numerically the decrease in species from Scotland and Norway to Iceland is 

 as striking, but relatively it is less well marked. Then, among the Icelandic 

 species only a minor fraction has spread to Greenland. Among these, two 

 {Cepaea hortensis and Oxychilus alliarius) have been regarded by modern 

 authors as probably introduced to Greenland. 



All definitely indigenous species in Iceland and Greenland must be regarded 

 as well adapted for passive dispersal. For two species in Iceland, Arion ater 

 and Arianta arbustorum, a spontaneous passive dispersal seems improbable. 

 However, Lohmander (1938), the author who has most thoroughly penetrated 

 the problem of Iceland's gastropod fauna, is of the opinion that for these very 

 species anthropochorous dispersal in historical time is a probable alternative. 

 A random, passive dispersal is also indicated by the absence of certain species, 

 which, as Lohmander assumes, are well able to endure Icelandic conditions. 

 For the idea of a Pleistocene landbridge to Iceland, and subsequent refugia, 

 the land Gastropoda offer no positive evidence. The nearest known peri- 

 glacial deposit of non-marine MoUusca is that of Lea Valley, England 

 (Kennard and Woodward, 1912), the age of which has recently been dated to 

 28,000 ± 1500 years, or early main Wurm (Godwin and WiUis, 1960). It 

 represents a poor fauna, but some of the species are definitely not Arctic 

 (Piipilla muscorum, various Succinea species, also some of the freshwater 

 forms). If existing, an Icelandic refugial fauna most probably would have 

 been still more depauperated. The presence of some remarkably demanding 

 forms (the Vitrea species, and Arion intermedius) suggests that the Post- 

 glacial Hypsithermal was of outstanding importance for the establishment 

 of the gastropod fauna in Iceland. 



Against this general background concerning the distribution of land 

 Gastropoda in the North Atlantic region, the presence of the European 

 Cepaea hortensis and Limax marginatus in northeastern North America is 

 very puzzling. American authors especially have been inclined to regard them 

 as indigenous. L. marginatus seems to be limited to a very narrow strip in 

 Newfoundland, whereas C hortensis has a rather extensive and relatively 

 continuous distribution from Newfoundland to New England. The conclusive 

 arguments have been concentrated in the discussion on the latter species. 



The opinion that C. hortensis has an indigenous distribution in America 

 in based upon observations by different workers who have studied its occur- 

 rence in natural habitats and pointed out that this species, contrary to 

 definitely anthropochorous species, seems to have a continuous, "mature" 



