The notch at the base of the shells of this family is a kind of groove
bent backwards and upwards, but not properly forming a canal, all
the genera have an operculum. The name Purpurifera has been given to
these Trachelipodes, because some of the genera contain in a peculiar
reservoir the colouring-matter with which the Romans, and other ancient
nations, dyed their beautiful and well-known purple, which was so much
in use before the discovery of cochineal.