Plastromancy is a form of divination using the plastron, or undershell of a turtle. It was mainly used in ancient China, but gradually fell out of use during the Later Han dynasty. The plastron was removed by the diviner on an auspicious day and then consulted by applying the tip of a hot metal bar to the shell and examining the crack marks caused by the hot metal. The conclusions were then inscribed in one or two lines beside the spot. Thousands of these inscribed shell fragments have been found in archaeological sites, particularly from the Shang dynasty. The inscriptions on the fragments have taught scholars much about the early development of Chinese writing.