MOLLUSCAN FAUNA OF THE BRITISH ISLES. 
401 
Atlantic, and on the east by the Ural Mountains and the Caucasus. 
The characteristic Helicine inhabitants are undoubtedly the Penta- 
trenia, a group embracing our familiar species Helix aspersa, Helix 
nemoralis, etc., which are the most highly organized Helices upon the 
globe, and possess a pre-eminent power of adaptability to novel con- 
ditions and an ability to prosper under comparatively unfavourable 
conditions. Although it is probable that the tertiary molluscan remains 
are usually correctly referable to the groups still inhabiting their 
respective districts, yet in this sub-region the strata of more ancient 
periods would, if preserved, probably display relics of more generalized 
t 3 ^es from which the widely distributed and more primitive groups 
have been derived. 
The British Isles belong to the European sub-region, and their 
molluscan fauna has been derived therefrom prior to the formation 
of the English Channel, when there was an actual connection at one 
or more points, the early isolation of this country doubtless contri- 
buting in some degree to the paucity of our fauna in comparison with 
that of the continent, although had the connection still e.xisted we 
must still have looked for diminished numbers of species and indi- 
viduals as we approach the north and become more remote from the 
centre of dispersion. 
The total number of terrestrial mollusca inhabiting these islands is 
127, of which 82 are land shells and 45 freshwater ; of this number 
Ireland possesses 105 species, of which 69 are land and 36 freshwater 
shells. Its early separation from Great Britain has prevented the 
influx of a few other species which exist in the north-western counties 
of England. Scotland, though continuous with England, has not 
favourable geological or physical features, and its geographical ex- 
tension northwards also acts prejudicially upon the development and 
exten.sion of many forms of molluscan life, so that we find only 102 
species resident therein, of which 68 are land and 34 freshwater, and 
these are mostly restricted to its more southern parts. 
The present fauna of these isles consists of a number of more or 
less archaic or simpler forms and a series of improved or stronger 
races or species, these becoming evolved by the competition and 
struggles for existence continually taking place, more especially in 
the extensive, densely inhabited and therefore more competitive areas, 
and with the more highly developed organisms— the stronger races 
which arise being more exactly adapted to the environment and also 
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6/12/1900 
