This spiral shell fossil from the Miocene Epoch was found in Okayama Prefecture. The shell has a characteristic form with a number of spines. Vicarya is a now extinct speices that flourished during the Miocene Epoch of some 1.5 million years ago. They are believed to have lived in inner bays. The fossil is used as an index fossil for the comparison of geographic strata. Fossils where the inside has metamorphosed into opal were found here, and are much-prized specimines with a nickname of "tsuki-no-osagari."