crown shell
now rare
noun
Any of various marine shells thought to resemble a crown (frequently with distinguishing word indicating the origin of the shell or the shape of the crown); (in later use) specifically the shell of any whelk or conch of the genus Melongena, found off the coasts of tropical America.
Origin
Mid 18th century; earliest use found in Robert Dodsley (1704–1764), bookseller and writer.