On Periploma mitsuganoense Araki (Bivalvia: Mollusca) from the Miocene Bihoku Group in Niimi City, Okayama Prefecture, southwest Japan ―with special reference to it’s paleogeographic significance―

In this paper, we deal with re-description of Periploma mitsuganoense Araki and it’s morphological variation and significance of the molluscan fauna of it from the Miocene Bihoku Group in Niimi City, Okayama Prefecture, Southwest Japan. Moreover, the paleogeographic significance of this species is analyzed. The obtained results are summarized as follows: 1. The morphological variation of Periploma mitsuganoense Araki has a wide range on the basis of the analysis of the morphological outline. 2. The occurrence of P. mitsuganoense Araki from the Pectinid fauna is the first record in the West Setouchi Geological Province. 3. It is presumable that P. mitsuganoense Araki is an endemic species in the First Setouchi Geological Province from a view point of it’s spatial distribution.