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any of several marine invertebrates of the class Bivalvia belonging to the family Anomiidae. In most species of these oysterlike bivalves, one valve (i.e., half) of the shell is closely appressed to a rock surface and has a large hole or embayment in its wall through which a calcified byssus (tuft of horny threads) attaches to the rock and thus anchors the animal. The shell's upper valve, though it is more convex than the lower, is usually irregular in shape and follows the configuration of the rock surface