Like other seashells, musselshells are made out of three main layers:

• the periostracum: the dark outer protective film that covers the shell, made of conchiolin protein;
• the ostracum: the prismatic calcareous part of the shell, composed of CaCO₃ and protein;
• the hypostracum: the inner iridescent part known as nacre, composed of alternating layers of aragonite platelet and protein

+ the byssus: the bundle of filaments (beard) through which the mussel sticks to a solid surface.