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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 as a coating on the body whorl near the aperture of the shell (i.e. a parietal callus or columellar callus). It is a hardened deposit of enamel, which varies in coloration and size depending on the species

A funiculus (plural: funiculi) is a narrow ridge of callus spiraling from the upper lip into the umbilicus.

A callus exists in the shells of various species of gastropods (snails) and also in the shells of several species of Nautilus, a cephalopod.

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.

References

1. "MOLLUSCAN GLOSSARY".
2. "Identifying Molluscs".
3. Definitions of terms for parts of gastropod shell

v t e Cephalopod anatomy
Shell	Types External Ammonoid shell Argonautid eggcase Nautiloid shell Orthocone Internal Belemnoid guard Cirrate shell Cuttlebone Gladius (pen) Spirula shell Stylet Features External Aperture Apex Callus Lirae Periostracum Sculpture Spire Sutures Umbilicus Internal Body chamber Camerae Nacre Phragmocone Septa Siphuncle Whorls
Mantle & funnel	External anatomy Dermal structures Fins (wings) Flotation devices ("secondary fins") Funnel–mantle locking apparatus Tail Internal anatomy Branchial hearts Ctenidia (gills) Hepatopancreas (digestive gland) Ink sac and ink Needham's sac Nephridia ("kidneys") Nidamental glands Osphradium Pericardial glands
Head & limbs	Brachial crown Arms Hectocotylus Tentacles Carpus Dactylus Manus Suckers and hooks Buccal region Aptychus Beak Odontophore Radula Spadix and antispadix Occipital region Nuchal crest (occipital crest) Nuchal folds (occipital folds) Occipital membrane Olfactory organ Nuchal organ Other parts Eyes Statocysts
General	Chromatophores Photophores Nervous system Squid giant axon Squid giant synapse
Developmental stages: Spawn → Paralarva (Doratopsis stage) → Juvenile → Subadult → Adult • Egg fossils • Protoconch (embryonic shell)

• Mollusc shells