Shell Size and Natural Selection in Cepaea nemoralis
Like many other helicid snails Cepaea nemoralis grows to a definitive adult size, at which point a lip is laid down on the edge of the mouth of the shell. The average maximum shell diameter over most of the range of the species is 20 to 22 mm, but in some regions, notably the Pyrenees, Cantabria and parts of Ireland, there are large-shelled races that reach a mean size of 24 to 30 mm. This geographical distribution has a parallel in the related species Arianta arbustorum, which is for the most part slightly smaller than the typical C. nemoralis but which reaches extremely large dimensions in some mountainous localities (see Cook 1965 where data of C. Oldham are presented). In parts of the range of C. nemoralis where dimensions fall within the typical limits there may be striking changes in mean breadth over short distances in uniform habitats (for example, Wolda 1969).
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