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.