The Miocene deposits distributed in the San-in district of west Japan are called the Iwami and the Izumo Groups in ascending order. From oldest to youngest the Iwami Group is subdivided into the Hata, Kawai, Kuri and Omori Formations. The Izumo Group is also subdivided into three units, the Kimachi, Fujina and Matsue Formations in ascending order.
In this sequence, there are two horizons which contain abundant marine molluscan fossils. The lower horizon is in the Kawai and Kuri Formations and their correlatives, which yield molluscs of a warm sea type correlated with the Kadonosawa Fauna of late Early Miocene to early Middle Miocene of northeast Japan. The upper horizon is in the Fujina and Matsue Formations and their correlatives, which yield cool water molluscs and relics of warm current species, correlative with the Shiobara-type Fauna of Middle to Late Miocene of northeast Japan.
As volcanic rocks dominate the remaining formations, marine mollusca in other horizons are rare.
The fossil locality reported in this paper is situated stratigraphically between the Kuri and the Fujina Formations, thus providing information on the change of marine molluscan faunas in this district.
Before entering into the discussion, the writer wishes to thank Dr. Paul Morris of Sydny University for improving the English.