Helmet-shell

Helmet-shell (Cassis), a genus of gasteropods, type of a family (Cassidæ), the members of which are somewhat whelk-like, and have thick, heavy shells, with bold ridges, a short spire and a long aperture, the outer lip toothed, the canal recurved. Numerous species, amounting to about fifty if we include closely allied genera such as Cassidaria, occur in the warmer seas. As the shells are made up of differently-coloured layers, they are much used for the manufacture of Cameos (q.v.). The species most used is the large Black Helmet (C. madagascarensis), sometimes almost a foot long, with a whitish outer and black inner layer.