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 inwluta^ 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^ Limnceus glaber^ AmpJiipeplea 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- 
