Biostratigraphical and Paleontological Studies on the Endothyroid Foraminifera from the Atetsu Limestone Plateau, Okayama Prefecture, Japan

Biostratigraphical study of the Lower Carboniferous through the endothyroid Foraminifera, has very recently been advanced by some paleontologists of North America and Russia. But their biozones are proposed only by E. J. ZELLER (1957) who has studied endothyroids from the Mississippian rocks of the Cordilleran region, North America. In the Atetsu limestone plateau the Carboniferous deposits are named Mitsudô group by the writer, and further subdisivisible into two formations, i. e. the lower, Nagoe formation of Mississippian and the upper, Kodani formation of Pennsylvanian in age. By careful thin section study of samples from the lower Nagoe formation of the Morikuni-Kodani measured section and two other adjacent ones in this area, the present writer recognizes the following five foraminiferal fossil zones in ascending order ; 1) Plectogyra communis zone, 2) Plectogyra primaeva zone, 3) Endothyra spiroides zone, 4) Endothyra symmetrica zone and 5) Atetsuella meandera zone. The endothyroid faunal succession of the inter-mediate three zones is very similar to that of the Osagian to the Upper Meramecian faunal zones proposed by E. J. ZELLER stated above, and moreover both species of Plectogyra communis and P. primaeva from the lowest two zones were described by D. M. RAUSER-CHERNOUSSOVA (1936) from the Lower Tournaisian in the central Kazakhstan, Russia. However, the lower part of the Endothyra spiroides zone yields such coral forms as Hexaphyllia sp. and Siphonodendron sp. generally thought to belong to the Upper Viséan (Chesterian) Onimaru type fauna which has never been reported from the Carboniferous limestone of Chûgoku. Al-though it has been believed generally that Profusulinella follows Millerella in phylogenic position of Fusu-linidae, the present writer discovered Profusulinella from the uppermost limestone of the Nagoe formation underlying perhaps disconformablly the lowest part of the Pennsylvanian Kodani formation in which Millerella appears first. The fact that a distinct abrupt change in faunas is recognizable between the upper-most Atetsuella meandera zone of the Nagoe formation and the overlying Pennsylvanian Kodani formation may stratigraphically indicate there a noticeable disconformity between the Mississippian and the Pennsylvanian deposits. A new genus Atetsuella, which is an intermediate type of Fusulinidae and Endothy-ridae, two new species of Atetsuella, three new species of a new genus Paraplectogyra belonging to the Endo-thyridae, and a new species of Granuliferella are described.