It is worth commenting that we aren't talking about the mussel appreciated in kitchens that everybody knows today. Those are shells from a different and freshwater species called 'painter's palette'. A dull brown, medium sized shell with a glossy and pure white inner face. Being common and easily available in most european rivers was a cheap and perfect solution for mixing oil colors in the, plastics-free, XVII siecle.
It is decreasing currently and even critically endangered in some european countries, but still remains relatively common in Holland. Can be found from Iberia to Russia and also in UK.