The Scallop Shell (Rheumaptera undulata or Hydria undulata) is a moth of the Geometridae family. It is found in most of the Palearctic ecozone and North America. The wingspan is 25–30 mm.The ground colour of the forewings is pale.The cross-lines are darker, wavy, evenly spaced and numerous. The outer margin is pale brown with a scalloped fringe. The hindwings resemble the forewings but are paler. There is one generation per year with adults on wing from the end of May to August in Europe.