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Above the mouth is the lid (operculum), which varies in size from a small narrow process to a large heartshaped expansion.
Spire of shell much reduced; two bipectinate ctenidia, the right being the smaller; no operculum.
Very usually, but not universally, the metapodium carries an operculum.
Cyclostoma, shell turbinated, operculum calcareous, British.
Foot divided into two, posterior half bearing the operculum; a wide epipodial velum; shell turbinated.