Arcusidiscus oppositus gen. et sp. nov. is described from the early Miocene sediments of the Boso Peninsula, central Japan. In addition to those in a marginal ring, there are three to six rimoportulae aligned in short arcs. Their location away from the valvar margin distinguishes this new genus from those with saucer-shaped valves.

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A molecular analysis of 14 species belonging to Pinnularia and Caloneis has been conducted with three molecular markers. Neither genus is monophyletic, thus supporting earlier contentions that the genera could not be separated. However, the molecular analysis does reveal two clades, which correspond to the division of the two genera following Krammer & Lange-Bertalot, using the degree of opening of alveoli as a criterion for separating species in the two genera. More analyses should be done with species from these genera to continue to verify that this is the morphological feature that divides the species before any taxonomic revision is made. Nevertheless, molecular data have indicated that separation of the species within these two genera is likely possible and that the separation is supported by morphological data, which will make the assignment of any existing or new species possible with traditional methods.

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In this paper we designate a lectotype and an epitype for the diatom species Synedra familiaris Kütz., an important species in ecological studies of freshwater attached algal communities. For this purpose, we have used a slide in Ktitzing's collection housed in the Natural History Museum, London (BM). The original slide from the type locality, was not found in the museum collection, and only one slide (BM18307) was present from another locality. No individual specimen could be identified as S. familiaris given its current concept. Three other possible taxa were described before S. familiaris. So the name S. familiaris should not be used with the current concept. We have also designated a lectotype for S. familiaris f. parva Grunow using an individual specimen from the holotype slide in the Natural History Museum, Vienna. This taxon no longer conforms to the current usage of the taxon S. familiaris, and so we propose a new combination Fragilaria parva (Grunow) Tuji et D. M. Williams comb. et stat. nov.

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Samples of epilithic diatom assemblages were collected from rivers, springs and a pond in Yaku-shima Island. The flora of Yaku-shima is described using these samples. Two new species, Achnanthidium ovatum and Gomphonema yakuensis, from River Anbou are described herein.

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Eunotia serra is characterized by a dorsal margin with 4 to 20 or more undulations. How this variation depends on cell size reduction has not been clarified thoroughly. Fourteen parameters characterizing the valve outline of E. serra were defined based on the arc-constitutive model described earlier by the present authors. Each parameter was evaluated using 147 valves collectedfrom a pond in west-central Japan. The crests of the undulation arcs of a given wave precisely defined a large envelope arc, except for a few undulation arcs near each end. The radius ofthe envelope arc and the number of undulation arcs both showed high positive correlations with valve length. In contrast, valve width and the parameters constituting the undulation arc were not highly correlated with valve length. Valve length and arc number both showed an almost log-normal distribution, but valve width showed a normal distribution. The distance between a terminal arc center and the adjoining undulation arc center displayed an approximately uniform distribution within the range of 5% to 80% of the mean distance between undulation arcs. This suggests a process whereby the terminal arc approaches the adjacent undulation arc with every cell division, and finally unites with it; continuation of this process results in a decrease in the number of undulations as cell size reduction proceeds. Both the crests and troughs of undulation arcs near the center of the dorsal margin lay indifferently about the transapical axis; furthermore, the shape of the two terminal arc neighborhoods of a give n valve is mutually uncorrelated. The alignment of the train of undulation arcs along the dorsal valve margin is random and generally not symmetrical with respect to the transapical axis.

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Investigation of four samples taken from an outcrop of the Ningyotoge Formation (Late Miocene to Pliocene), around the Ningyo Pass, between Okayama and Tottori Prefectures, Japan, revealed 101 taxa in 42 genera of fossil freshwater diatoms. Diatom assemblages of the four samples were dominated by centric diatoms (7 taxa) with a frequency of 56-82%, followed by araphid pennate diatoms (21 taxa) 7-18%, monoraphid pennate diatoms (13 taxa) 6-10% and biraphid pennate diatoms (60 taxa) 4-16%. Taxonomic information on 10 of the diatom taxa is reported here: Aulacoseira ambigua (Grunow) Simonsen, Aulacoseira sp., Discostella sp., Cocconeis placentula Ehrenb. var. lineata (Ehrenb. ) Van Heurck, Didymosphenia sp., Epithemia cistula (Ehrenb. ) Ralfs, Gomphonema sp., Melosira undulata (Ehrenb. ) Kütz., Synedra capitata Ehrenb. and Tetracyclus stellare var. eximius (Hérib. & Perag. ) Hust. Of particular interest is the presence of a Didymosphenia taxon, indicating that a cool temperate climate existed during the formation of the deposit. Six taxa of Tetracyclus occurred in the fossil diatom assemblage.

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We collected the attached algal samples in three streams and two spring waters near Numea, the capital of New Caledonia, late in March,2006. In this study, we performed the same sampling method used to collect attached algae on the river bed in Japan, in order to compare the two diatom assemblages. In the Numea samples, the five most dominant diatom taxa were Achnanthes minutissima var. scotica, Cymbella delicatula var. delicatula, C. falaisensis, C. latarea and Encyonema tenerum. These five taxa have never been recorded as dominant in attached diatom assemblages in Japan, however, attached diatom assemblages in streams with clean water are usually dominated by Achnanthes, Cymbella and Encyonema in both New Caledonia and Japan.

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The diatom flora of Kojorougaike Pond, located in the Hira Mountain Range in west-, central Honshu, Japan, was studied in April 2004. Diatoms were collectedfrom five different substrates, including dead monocotyledonous leaves, dead and living Sphagnum, mud and peat. A total of 38 diatom species belonging to 19 genera were identified and illustrated together with three unidentified species. The predominant genera were Pinnularia and Eunotia, represented by ten and seven species, respectively. While Pinnularia schoenfelderi Krammer was dominant on living Sphagnum, Kobayasiella micropunctata (H. Germ. )Lange-Bert. was dominant on dead ones. On the other substrata, Eunotia rhomboidea Hust. and Eunotia curvata (Kütz. ) Lagerst. were dominant.

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Didymosphenia geminata (Lyngb. ) Mart. Schmidt has been found in the River Izari near Lake Shikotsu, Hokkaido, Japan. The sampling point is truly riverine and so might be representive of its natural distribution. The ultrastructure of these individuals was compared with Icelandic and Mongolian individuals using an SEM. Though the densities of striae and areolae show much variation, the ultrastructure of the areolae are identical and so the individuals are thought to be part of the variation of a single taxon. Another small Didymosphenia taxon, which has one stigma, has very different form of areola from D. geminata.

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