Gastropoda and BivalviaBivalve, (class Bivalvia), any of more than 15,000 species of clams, oysters, mussels, scallops, and other members of the phylum Mollusca characterized by a shell that is divided from front to back into left and right valves. Gastropods are among the few groups of animals to have become successful in all three major habitats: the ocean, fresh waters, and land. |
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TrilobitesTrilobite, any member of a group of extinct fossil arthropods easily recognized by their distinctive three-lobed, three-segmented form. |