 Most cultured pearls today come from "Hyriopsis" species in Asia, or "Amblema" species in North America, both members of the related family Unionidae; pearls are also found within species in the genus "Unio".The interior of the shell of "Margaritifera margaritifera" has thick nacre (the inner mother of pearl layer of the shell).This species is capable of making fine-quality pearls, and was historically exploited in the search for pearls from wild sources.