Pseudoleyanella lunata Takano is a marine diatom with dorsiventral valves belonging to the family Cymatosiraceae. We studied the range of variation in valve morphology throughout the life cycle. We also observed the chloroplast division of this species. In large cells, the valves were narrowly lanceolate, slightly capitate at the apices, and asymmetrical with respect to the apical plane, i.e. with almost straight and convex margins on the ventral and dorsal sides, respectively. During cell size reduction, the valves gradually lost their dorsiventral nature, and eventually became almost circular. Although P. lunata was rectangular in girdle view in small cells, large vegetative cells, particularly those generated soon after auxosporulation, were slightly bent, as in Leyanella Hasle et al. However, in contrast to Leyanella, P. lunata lacked both pili and tubular processes at all stages of its life cycle. In phylogenetic analyses (SSU rDNA and rbcL), P. lunata was sister to Leyanella. We also observed the whole structure of the diatotepum which was a sheet-like structure underlying each theca. Under the transmission electron microscope, dot-like marks with high electron density were observed: their position and pattern corresponded to valve poroids. In addition, the diatotepum bore electron-dense lines corresponding to the internal sutures between the girdle bands.

Mesodictyon japonicum Yanagisawa & H. Tanaka is an extinct centric diatom described and reported from upper Miocene marine sediments. The species had been presumed to be a freshwater species because of the occurrence pattern similar to that of accompanying Aulacoseira species which may be transported from inland lakes to marine environments. However, the possibility that this species was a marine species can not be completely ruled out. In the course of continued study, we have found the mass occurrence of M. japonicum with other freshwater diatoms in the lacustrine deposits of the late Miocene Takatama Caldera (8–9 Ma in age) in Koriyama City, Fukushima Prefecture, and thus confirmed that this species was a freshwater diatom.

We investigate the diatom flora of Yamamuro Moor, a small intermediate moor formed in a valley bottom in the northeast of Lake Biwa in Shiga Prefecture, central Japan. It is a true Sphagnum moor with a thin peat bed despite its situation in the warm-temperate zone. A field survey and sampling took place on 21 April 2007. The water was slightly acidic (pH 5.3–5.8) with low electrical conductivity (2.8–3.4 mS m^-1). From 24 diatom samples collected from the surfaces of spermatophytes, Sphagnum spp., and peat in the moor, 65 diatom taxa belonging to 22 genera were found. These are all listed and illustrated here, although six of them have not been identified to species. In terms of taxa richness, the dominant genus was Eunotia, represented by 16 taxa, followed by Pinnularia, with eight taxa.

Thalassiosira laevis, a rare nanoplanktonic diatom species described from Xiamen Harbor (East China Sea), is for the first time recorded in the South Atlantic Ocean from fossil and modern samples. It was an important fossil diatom indicating sea level changes in the assemblage recovered from a Late Holocene sequence from the Brazilian coast (coastal plain of São Paulo State) reaching an abundance of 53% at about 5000 yr BP. In modern samples, it was a rare species found in plankton and exposed sand sediments from dissipative marine sandy beaches of Rio Grande do Sul State (Praia Azul, Tramandaí and Cassino) and a bay beach in Santa Catarina State (Zimbros). Distribution data shows Thalassiosira laevis as a widespread diatom living along coastal littorals and inlets close to estuarine area mouths.

Three Gomphopleura taxa, G. frickei, G. poretzkiae and G. cf. nobilis were found in one sample of diatomite of Setana, an Early Miocene sediment, from Hokkaido, Japan. The three taxa were observed using light and scanning electron microscopies, and their characteristics of shape, size, external opening of areolae, axial area, apical spine, apical pore field, longtudinal lines and raphe were documented and are described herein. The differences among the three taxa: G. frickei has a mostly parallel center G. poretzkiae has an expanded center and G. cf. nobilis has almost isopolar, lanceolate valve. Furthermore, G. cf. nobilis usually lacks marginal ridges, external openings of areolae are straight slit-shaped and longitudinal lines are near the valve shoulder.

A fossil diatom species, Stephanodiscus akutsui sp. nov., is described from Middle Pleistocene lacustrine sediments of the Miyajima Formation of the Shiobara Group in Tochigi Prefecture, central Japan. Stephanodiscus akutsui is characterized by a valve face with concentric undulation, a shallow mantle, bi- to four-seriate fascicles at the valve margin, zero to two valve face fultoportulae, a single rimoportula, and valves often linked by spines into chains. The combination of morphological characteristics of S. akutsui differs from other related Stephanodiscus species.