(formerly Hydrocaryaceae), a family of aquatic dicotyledonous plants that contains a single genus—Trapa (water chestnuts). The solitary flowers develop in the axils of floating leaves and are on short pedicels that rise above the water. There are four petals, four sepals, and four stamens. The one-seeded fruit usually has two or four hornlike processes. The seed has two cotyledons: one is large and fleshy, and the other small and scalelike. Water chestnuts grow in wind-protected freshwaters—oxbow lakes, river and lake backwaters, and small lakes. Fossils are known from the Late Cretaceous, and fruits are often found in Tertiary deposits.