A parietal callus is a feature of the shell

Gastropod shell

The gastropod shell is a shell which is part of the body of a gastropod or snail, one kind of mollusc. The gastropod shell is an external skeleton or exoskeleton, which serves not only for muscle attachment, but also for protection from predators and from mechanical damage...

 anatomy of some groups of snail

Snail

Snail is a common name applied to most of the members of the molluscan class Gastropoda that have coiled shells in the adult stage. When the word is used in its most general sense, it includes sea snails, land snails and freshwater snails. The word snail without any qualifier is however more often...

s, i.e. gastropods. It is a thickened calcareous

Calcareous

Calcareous is an adjective meaning mostly or partly composed of calcium carbonate, in other words, containing lime or being chalky. The term is used in a wide variety of scientific disciplines.-In zoology:...

 deposit which may be present on the parietal wall of the aperture

Aperture (mollusc)

The aperture is an opening in certain kinds of mollusc shells: it is the main opening of the shell, where part of the body of the animal emerges for locomotion, feeding, etc....

 of the adult shell. The parietal wall is the margin of the aperture and part of the wall of the body whorl that is closest to the columella. The callus is often smooth and glossy, but can also be decorated with raised ribs or wrinkles.

This feature is found in such marine families as Ranellidae

Ranellidae

Ranellidae, common name the triton shells or tritons, is a taxonomic family of small to very large predatory sea snails, marine gastropod mollusks in the clade Littorinimorpha.-Subfamilies:...

 (pictured), Cassidae, Nassariidae

Nassariidae

Nassariidae, common name nassa mud snails , or dog whelks , are a taxonomic family of small to medium-sized sea snails, marine gastropod mollusks in the clade Neogastropoda.-Shell description:...

, Ringiculidae

Ringiculidae

Ringiculidae are a family of minute deep water sea snails or micromolluscs, marine gastropod mollusks or micromollusks in the informal group Lower Heterobranchia.Ringiculidae is the only family within the superfamily Ringiculoidea....

 and others. It is also found in some families of land snails, terrestrial pulmonates.