Plastromancy is a form of divination using the plastron, or undershell of a turtle.
Derived from the English plastron and Greek manteia ('prophecy')
It was mainly used in ancient China, especially the Shang dynasty but gradually fell out of use during the Later or Eastern Hàn dynasty. The earliest use of turtle shells can be traced back from the archaeological site in Jiahu. The shells, containing small pebbles of various size, color, and quantity, were drilled with small holes, suggesting that each pair of them was tied together originally.
Similar finds have also been found in the Dawenkou burial sites of about 4000–3000 BC, as well as in Henan, Sichuan, Jiangsu and Shaanxi. The turtle-shell shakers for the most part are made of the shell of land turtles, identified as Cuora flavomarginata. These rattles have been unearthed in quantity, with 70 being found in the Jiahu site, and another 52 being found in the Dawenkou culture sites at Dadunzi, Jiangsu, and type site, Liulin and Wangyin in Shandong. Archaeologists believe that these shells were used either as rattles in ceremonial dances, shamantic healing tools or ritual paraphernalia for divination purposes.
The plastrons were first prepared through applying the tip of a hot metal bar to the interior (flesh-side) surface. During the divination session, the diviner then applied a heat source to the pits, causing cracks to appear on the exterior surface. These cracks were then read as portents or answers to the topic of divination at hand.
During the Shang dynasty, the date, diviner's name and topic of divination were generally written on the plastron, sometimes along with prognostications. Occasionally, verifications about the event or day in question were later added as well. This record was then usually incised into the plastron's surface.
How exactly the cracks were interpreted is not known. The topic of divination was raised multiple times, and often in different ways, such as in the negative, or by changing the date being divined about. One oracle bone might be used for one session, or for many, and one session could be recorded on a number of bones.
The scapulae of oxen and other animals, and sometimes other bones, and even animal or human skulls were used for such osteomancy (divination using bones).