rudist bivalves

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rudist bivalves An extinct group of coraland horn-shaped bivalves (Bivalvia, subclass Heterodonta). They have a variable morphology and many species are not easily seen to be bivalves. They are adapted to a sedentary mode of life. Some forms have a larger lower left valve and an upper right valve that forms a flat lid; other forms have a larger lower right valve. The lower valve is often very thick and may have a complex wall structure. There is a tendency to develop a cylindrical and coral-like form. Rudist bivalves are found in Cretaceous rocks, sometimes forming extensive mounds.