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a genus of freshwater bivalve mollusks. The two elongate or oval halves of the shell reach a length of 13.7 cm and are clamped shut by two pairs of short front teeth (one pair on each side) and by three long rear teeth—one on the right and one on the left. The mollusks live in rivers and along the shores of lakes. They feed on organic matter suspended in the water and move along the river or lake bottom by pushing with their wedge-shaped foot. The sexes are separate. The female carries the fertilized eggs, as many as 400,000, in her gills. The eggs develop into glochidia—larvae that parasitize fish. Modern Unio is represented by several species, distributed in Europe, Africa, and Asia. Sources conflict as to the number of species in the USSR. Estimates range from two to seven species. The largest and thickest shells are used for making mother-of-pearl buttons.