Oki islands have been familiar to the Japanese geologists as the working area of petrological studies of alkaline rocks, while the biostratigraphic works have not so developed as to establish the whole successions. It is the upper Miocene to be confirmed as the oldest horizon by marine fauna. TOMITA (1936) first reported the occurrence of Cardium shinjiense, Crenella paeramaena, Mya arenaria, Pecten swiftii, P. yessoensis, Natica janthostoma, Chrysodomus despectus and Thyasira bisecta from his Dogo group. Later, foraminifers such as Cibicides lobatulus, Globigerina bulloides, Uvigerina subperegrina, Epistominella japonica and so on, were found out by TAI (1954) from the same horizon. Since then, these fossils have been regarded as the indicators of the upper Miocene, and the whole successions of volcanic activities have been made on this paleontologic basis
Accordingly, a new middle Miocene fauna described below as Kumi fauna is quite noteworthy, because it means the first evidence of the middle Miocene transgression in Oki islands. It is, moreover, interesting in the paleo-ecological considerations that this new fauna is composed mainly of rocky bottom inhabitants in the warm current.
Before entering into the description, the authors wish to express their hearty thanks to Mr. S. KASAMATSU of Education Office of Saigo-cho for the information of fossil locality, and Dr. M. OMORI of Azabu Veterinary College and Dr. J. ITOIGAWA Of Nagoya University for their kind suggestions on paleontology.