A callus is an anatomical feature that exists in some mollusk shells, a thickened area of shell material that can partly or completely cover the umbilicus, or can be located on the body whorl near the aperture of the shell (i.e. a parietal callus or columellar callus).
A callum is an anatomical feature of some mature bivalve shells of species in the family Pholadidae, the piddocks. The callum is an area of shell material that fills the gap between the two valves.
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