Umbilicus (mollusc)

The word is also applied to the depressed central area on the planispiral coiled shells of Nautilus species and fossil ammonites.

The spirally coiled whorls of gastropod shells frequently connect to each other by their inner sides, during the natural course of its formation.

On the other hand, if this connection is less intense, a hollow space inside the whorls may result, with an opening to the outside at the shell's base.

[1] Another way of characterizing the umbilicus in gastropods is as the hole around which the inner surface of the shell is coiled, when that space is not filled by a columella.

Shells with a conspicuous umbilicus are always orthostylic, i.e. they have a poorly developed columella.

The umbilicus is clearly visible on the underside of the left shell of these three shells of Xerolenta obvia
The wide and deep umbilicus of Cittarium pica is easily visible on the underside of the shell.
The shell of Neverita josephinia has an umbilical callus which almost completely fills the umbilicus, leaving only a groove, and a chink on one side.
Three views of an ammonite shell, clearly showing the central umbilicus.
The umbilicus of the shell of the nautilus Allonautilus scrobiculatus