The shell is solid, swollen, approximately oblong or irregularly triangular in shape, with blunt umbones. The shell is dark blue or purple in colour, however the perisostracum gives adults a glossy yellow or dark brown appearance. In young animals the shell appears bluish and the periostracum is extended into long, smooth spines. The shell bears clear growth lines, and a sculpture of fine concentric lines and ridges. The inside of the shell is white, with a wide pallial line, a large anterior adductor muscle scar and small posterior adductor muscle scar. Adults are usually more than 10cm in length, although very large specimens may reach up to 22cm.Distinguished from the common mussel (Mytilus edulis) by the blunter umbones, which are not quite anterior due to the extension of the shell margin in Modiolus modiolus.