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Sediment

If the shell incorporates "grains of sediment", then does that imply, that the shell was formed in situ, in the sediment, by a burrowing creature?

No, the creature may well have "picked up" the grains from the sea floor and stuck them into a sticky matrix, like some modern foramanifera. Martin ( Smith609 –  Talk) 13:25, 2 March 2011 (UTC) reply