Umbilicus (mollusc)

The umbilicus of a shell is the axially aligned, hollow cone-shaped space within the whorls of a coiled mollusc shell. The term umbilicus is often used in descriptions of gastropod shells, i.e. it is a feature present on the ventral (or under) side of many (but not all) snail shells, including some species of sea snails, land snails, and freshwater snails.

The word is also applied to the depressed central area on the planispiral coiled shells of Nautilus species and fossil ammonites. (These are not gastropods, but shelled cephalopods.)