Tertiariopsis undulata sp. nov., a fossil freshwater diatom species from the lower Pliocene marine sediments, northeastern Japan

抄録

The lower Pliocene interval of the marine Tentokuji Formation distributed near Nangai Dam, Daisen City, Akita Prefecture, northeastern Japan, was found to contain fossil lacustrine diatom Tertiariopsis costata along with an unknown Tertiariopsis taxon which in all probability was transported from inland lakes to marine environments. This paper describes this unknown taxon as Tertiariopsis undulata H.Tanaka & Yanagisawa sp. nov. The species exhibits the main characteristics of the genus Tertiariopsis: a sessile rimoportula on the mantle, mantle fultoportulae with three satellite pores, mantle fultoportulae and rimoportula covered by a marginal lamina, and distinct sections of mantle separated by hyaline strips. The new species, however, differs from other Tertiariopsis species in having a transversely undulate valve face. The species resembles T. costata as both species possess ridge-like thin mantle costae located on the inner mantle between the valve rim and mantle fultoportulae, but differ in valve face features: T. undulata has an undulated valve face, while T. costata is typified by an almost flat valve face.