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The Rostroconchia is a class of extinct mollusks dating from the early Cambrian to the late Permian. They were initially thought to be bivalves, but were later given their own class. They typically have a single, pseudobivalved shell enclosing the mantle and muscular foot. The anterior part of the shell probably pointed downward and had a gap from which the foot could probably emerge. Rostroconchs probably lived a sedentary semi-infaunal lifestyle. There were probably more than 1,000 species of members of this class.