M.

Margin, the whole circumference or outline of the shell in bivalves.

Marginated, having a prominent margin or border.

Membrane, a web of several sorts of fibres.

Membranaceous, consisting of membranes.

Mottled, clouded or spotted with various colours.

Mucronate, ending in a sharp rigid point.

Multilocular, many-chambered, consisting of several divisions.

Muricated, clothed with sharp spines.