An order containing seven superfamilies of marine and brackish bivalves with byssal attachment at some time in their geological history (the Ostreacea are exceptional, with the lower valve attached by byssal cement at settlement, and with no record of ancestral attachment by byssal threads). Byssal attachment is associated with departure from the ancestral isomyarian form; Mytilacea and Pinnacea are heteromyarian, with a small anterior adductor muscle; others are monomyarian, with one central posterior adductor. Byssal attachment is lost in some: Placuna is free-living; Pecten maximus can right itself if inverted; Chlamys can swim to escape from starfish; some Lima can swim, while the tropical Amusium swims strongly. Pearls are obtained from Pinctada (Pteriacea).