The shell of an adult Modiolus barbatus can be as long as. The shape is elongated and ovoid or rhomboidal with a curved or obtuse dorsal margin. The outer surface of the shell is dark brown, while the inside of the valves is blue or slightly iridescent gray. These mussles are characterized by a periostracum with long, stiff, flat bristles. They are attached with strongbyssus to the underground.