We report on the occurrence and valve morphology of two fossil diatom taxa assigned to the genus Lindavia from the Pleistocene Hikita Formation in the Urizura Hills, Hitachiomiya City, Ibaraki Prefecture, central Japan. Lindavia omarensis (=Pliocaenicus omarensis), which was previously reported from the Upper Pliocene to the Lower Pleistocene in Japan, was identified from a wide stratigraphic interval in the Hikita Formation. Lindavia rhomboideoelliptica var. rounda, which was previously recognized from the Middle Pleistocene in Japan, occurred exclusively from the upper part of the Hikita Formation. Co-occurrence of L. omarensis and L. rhomboideoellitica var. rouda in the upper part of the Hikita Formation is discordant with the previously known occurrence range of the two taxa, and this suggests that both or either of the two taxa possibly reworked fossil. However, the depositional age of the Hkita Formation is not well determined. It would be necessary to examine the age of the Hikita Formation in order to clarify the biostratigraphy of both taxa in this formation.

The depositional age of the Wada Formation in Kamiwada, Takahata Town, Higashiokitama District, Yamagata Prefecture, Japan, has been unknown. Correlation with other formations in the surrounding area has been difficult because the Wada Formation occurs only in the Wada Basin. To estimate the depositional age of the formation, diatom fossils in tuffaceous mudstone taken from an outcrop located along the Oguro River were identified. The fossil diatom flora was dominated by two taxa, Cyclotella iris Brun & Héribaud sensu lato (s.l.) and Mesodictyopsis miyatanus Tanaka, with rarer Fragilaria and other araphid taxa forming ribbon-like colonies. Based on age ranges of the occurrences of C. iris s.l. and Mesodictyopsis species in the Japanese Islands, the depositional age of the Wada Formation is identified as latest Miocene or early Pliocene, not much earlier than 6 Ma.

Diatom assemblages, composed of totally 171 floral taxa from Holocene bore-hole sediment in the Iriai Lowland, Oga Peninsula, Akita Pref., northeastern Japan, are presented here with photomicrographs identified under light microscopy. Biostratigraphy of those diatoms is confirmed for a refined reconstruction of the past burial history of the lowland.