Lysimachia nummularia
A very low creeping plant with a stem up to 2' long. The short-stalked, small, round, shiny leaves are paired along the stem. The yellow, 5-petaled 1" flowers are often in pairs on slender stalks from the leaf axils and have sepals half as long as the petals. Blooms June-August in moist areas, roadsides, fields, lawns. A native of Europe.