Parietal callus
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A parietal callus is a feature that can be found on the shell of a some snails, especially certain sea snails, marine gastropod mollusks.
This callus is a thickened calcareous deposit on the inner wall of the aperture of the gastropod shell, the wall nearest to the columella. The callus is usually smooth and glossy.
This feature is found in such marine families as the Ranellidae as pictured here, the Cassidae, the Nassariidae and others.