(The elongated conical shell of) any of various small gastropod molluscs of the family Pyramidellidae, which are mainly ectoparasites of sedentary invertebrates such as bivalves and tube worms.
Origin
Mid 19th century.
Definition of pyramid shell in US English:
pyramid shell
noun
(The elongated conical shell of) any of various small gastropod molluscs of the family Pyramidellidae, which are mainly ectoparasites of sedentary invertebrates such as bivalves and tube worms.