Bulletin of the Mizunami Fossil Museum

Monograph of the Mizunami Fossil Museum No. 6

Nine molluscan assemblages were discriminated in the Paleogene Urahoro Group, and 16 in the Omagari Formation and 12 in the Charo and Nuibetsu fromations of the Paleogene Ombetsu Group. Molluscan assemblages of both groups indicate such various environments as brackish or fresh water, brackish-water and near shore shallow marine and shallow to deep water. Most shallow and deep water assemblages are characterized by dominant and associated occurrences of genera, Acila, Yoldia, Portlandia, Cyclocardia, Dentalium, Turritella, Orectospira,and Eocylichna. These assemblages are also known from the Paleogene Poronai Fauna of north Japan (Mizuno, 1964), and are comparable in the fauna! composition to the "C Assemblage" of the Paleogene Setogawa stage in Shizuoka Prefecture, central Japan and to the fauna in the Paleogene Muro Group in Kii Peninsula of southwest Honshu, Japan (Mizuno, 1973).