GEOGRAPHICAL DISTRIBUTION. 299 



The facility with which these animals migrate and adopt a new 

 country, as proved by the above instances, by those cited at 

 p. 291., and by the naturalisation of the H. cantiana (130.) on 

 the banks of the Tyne, will always make the study of the geo- 

 graphical distribution of the terrestrial Mollusca difficult. Ac- 

 cording to the accounts of the American authors, the species 

 which have naturalised themselves retain their fondness for 

 trees and hedges and herbage, and keep themselves quite dis- 

 tinct from the forest-living species of America. 



There are one or two species whose distribution appears 

 more to depend on the nature of the country than the climate. 

 Thus, the Alasmodon elongatus is found in the mountain 

 streams of Wales, Cumberland, Scotland, and Ireland, and the 

 variety A. e. Roissyi is found in similar situations in Yorkshire 

 and Scotland. 



In Irish lakes there has been found by Mr. Harvey Amphi- 

 peplea involuta, which is very different from the English ones. 

 It is to be hoped that this is only a forerunner of several 

 other species which will hereafter be found in that very inter- 

 esting and but little investigated country. 



It is probable that many of the species may have a much more 

 extended range than is generally believed ; for had this sketch 

 been written a very few years ago, many of the species, such as 

 Helix fusca, H. depilata, Bulimus montana, Azeca tridens^ Pupa 

 edentula, Acme fusca, Linmceus glaber, Amphipeplea glutinosa^ 

 would have been inserted in the list of local species. The 

 latter, though found in Sweden and France, is not recorded as 

 a German species by Pfeiffer. Though very local where found, 

 these and other species similarly circumstanced have been 

 met with, dispersedly, in widely different parts of the islands. 



Besides fewer species being found in the northern parts of 

 our island than in the south, the specimens of the species are 

 said to be much more rare. This is partly owing to the rigours 

 of the climate, and partly to the country consisting of the older 

 geological formations, which are less favourable to the support 

 of these animals than the calcareous rock, which appears to be 

 their favourite habitation. 



M. D'Orbigny, who has paid great attention to the distribu- 



