Gastropods are any of a large class of mollusks (as snails) that have a muscular foot at the bottom and usually both a distinct head bearing sense organs and a spiral shell into which the body can be withdrawn.
Gastropods are very common along the London clay sites, they come in a very large varity of species with distinctive diagnostic properties.
When fresh they will still have a white shell but this is quite fragile and is not often found.
The are usually preseved in pyrite which will need treatment to prevent decay.
They can be found most commonly within pyrite shingle patches along the beach.
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