Abstract
A new fossil Thalassiosira species, T. mizunamiensis Yanagisawa sp. nov., is described from the early Middle Miocene Oidawara Formation in Mizunami area, central Japan. This species is characterized by a subcentral fultoportula in the middle of the radius, a single rimoportula located at margin, decreasing areolae size toward the margin and distinctly ribbed fairy broad rim. T. mizunamiensis occurs from several early Middle Miocene sequences in Japan, ranging in age from the upper part of the Denticulopsis praelauta Zone (NPD 3B) to the lower part of the D. lauta Zone (NPD 4A) of Akiba's (1986) Neogene North Pacific diatom zonation. It is of potential biostratigraphic value in this stratigraphic interval.
