; phylum Mollusca, class Bivalvia)
A subclass of bivalves, most of which are sedentary, epifaunal, and byssate or cemented. The most specialized genera have a reduced anterior adductor muscle and foot, and in some forms they are absent. The shells are variable in shape and composition; they may be aragonite, calcite, or both. They have a duplivincular ligament, unique to certain members of the subclass, and the dentition is variable. They first appeared in the Early Ordovician.
Subjects: Zoology and Animal Sciences.