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Comes in a range of colours, including piebald!
This specimen looks pale due to loss of the chitinous periostracum layer caused by weathering. This is not the same as a range of colours of the underlying shell itself.
Thanks Martin - a good response.
I spent time with a binoscope on the ones for my study. In the blotchy ones and those with a much paler complexion, I didn't note a peeling periostracum as a factor.
This though http://www.ispotnature.org/node/311278
might prove how UNobservant I am!
So, off I go tomorow to look at some of my marked ones and start to note seasonal periostracial changes.
Lat/Lng: 51.818724166667, -2.1609827777778
OS grid ref: SO889133