Mollusca (snails) are invertebrate animals that are ubiquitous in many terrestrial and aquatic habitats, both freshwater and saline. Mollusca are characterised by the shape and colour of their distinctive shells, which are formed of calcium carbonate.

Most mollusca are gastropods, that is, they have a single shell. Some, particularly marine types, such as oysters, cockles and mussels, are bivalves having an upper and lower shell. They preserve well in calcium-rich deposits, for example, tufa, a type of calcium deposit that is precipitated from limestone and found in various locations in Ireland.
The study of marine and land mollusca found sealed in archaeological or natural deposits can provide evidence of local site environment and, in the case of many marine mollusca, diet.

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