Aggericorona ponderosa gen. et sp. nov. and Lentiliforma flammifera gen. et sp. nov. are described as new late Miocene diatom taxa from the Morai Formation of western Hokkaido, Japan. A. ponderosa resembles Endictya, possessing a short cylindrical frustule composed of petri dish-shaped valves, which are loculate with extremely fine inner cribrate vela and outer foramina. Rimoportulae are located in a circle at the junction of the valve face and mantle. Lentiliforma flammifera resembles Pseudohyalodiscus, possessing a biconvex lens-shaped frustule with valves penetrated by numerous fine tubular poroid areolae. Small rimoportulae are arranged in a marginal ring as well as being scattered over the whole valve.

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Diatoms from three samples taken from Late Miocene sediment at three different outcrops along the Ebiya Gorge, Naru, Mitoku area in Misasa Town, Tottori Prefecture were investigated and authors found them to contain 45 taxa belonging to 27 genera, of which 41 taxa in 25 genera were identified. The diatom assemblage of the lowest sample (TOT-201) was dominated by Cyclotella kohsakaensis with a frequency of 73%, followed by Staurosirella lapponica at 11% and Diploneis subovalis at 3%. The diatom assemblages of the two upper samples (TOT-202,203) were very similar containing mostly Staurosirella lapponica with a frequency of 53-61%, followed by Aulacoseira italica at 10-14%, Pseudostaurosira brevistriata var. inflata at 2-6%, Mesodictyon sp. at 3-4% and Epithemia turgida at 2-4%. This report lists all the Late Miocene freshwater diatoms found in the three assemblages and notes the characteristics of five species.

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The fine structure of the marine diatom species Achnanthes longipes is presented. The valve shape is panduriform to linear-lanceolate, with biseriate striae on both valves; the cells have many disc-shaped plastids. This species forms colonies of long chains, usually with more than 10 individuals connected to each other. However, these colonies are not formed by terminal spines or marginal ridges on the ARV. Japanese specimens possess terminal orbiculi. As terminal orbiculi may appear as one of the structural patterns of large and modified cribra, we suggest that both individuals those forming terminal orbiculi on the ARV and those lacking them are classified as A. longipes.

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Japan has numerous irrigation ponds, however, few studies have investigated the diatom flora in them. Irrigation ponds are vulnerable to agricultural activity and urbanization because of their site location and small size. For these reasons, diatom communities in ponds might be affected strongly by recent anthropogenic influences. In this study, to clarify the diatom flora in the ponds and the historical changes in diatom communities, we investigated the diatom remains collected from sediments during the few past centuries in three irrigation ponds in Osaka Prefecture, Japan: Sayama-ike, Hataô-ike and Kodera-ike Ponds. Diatom remains were found in all samples throughout the sediment cores obtained from each pond. Altogether,46 genera and 176 diatom taxa (species, variety and forma) of the three ponds were identified. Among them, in particular in common with all ponds we identified three centric species, Aulacoseira subborealis, Cyclotella stelligera and C. pseudostelligera, which became dominant after the 1950s in Sayama-ike and Hataô-ike Ponds, although in Sayama-ike Pond A. subborealis had been temporarily do minated even before the 1950s. Meanwhile in Kodera-ike Pond, where eutrophication had progressed earlier than in the other two ponds, A. subborealis became predominant earlier before the 1950s, and C. pseudostelligera dominated both before and after the 1950s. Furthermore, in Sayama-ike and Kodera-ike Ponds, two common centric species, Aulacoseira granulata and Cyclotella meneghiniana, were identified. The latter species increased drastically after the 1950s. Present results suggest that these centric species appeared commonly in the three ponds as a result of recent environmental changes during the 20th Century.

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The wall ultrastructure of Raphidodiscus marylandicus T. Christian is observed with SEM on intact specimens from the marine Miocene Nabuto Formation, Boso Peninsula, central Japan. The valve consists of an almost circular valve face with a reduced plicate raphe situated diametrically, and an extremely narrow valve mantle. A thin plate of cribrum covers the whole of the valve. On the valve face, the central lanceolate region is alveolate, the pericentral field is coarsely loculate and the circumferential annular area is finely loculate. Each alveolus communicates with the adjacent loculus through a passag e pore.

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Effects of nitrogen on biomass accrual of benthic diatom assemblages were examined using indoor artificial streams supplied with spring water (NO₃-N,0.306 mg/1; PO₄-P,0.075 mg/1; SiO₂,35.3 mg/I; pH 7.4; conductivity,148 μS/cm) from the source of the Jigoku River, Nikko City, Japan. We manipulated the NO3-N concentration in the artificial streams at 0.306,0.781,1.586 and 2.349 mg/1 and investigated the temporal pattern of the biomass accrual in each stream to estimate the intrinsic growth rate and maximum biomass using the logistic growth equation. The results showed that there were no significant differences in the intrinsic growth rate and maximum biomass between the streams at the four different NO₃-N concentrations. As the concentration of phosphorus in spring water was greater than the concentration saturating benthic algal growth (0.025-0.050 mg/1), the ambient nitrogen concentration of 0.3 mg/1 is already growth-saturating for lotic benthic diatom assemblages.

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There are two freshwater environments which are considered to be severe conditions for the survival of living creatures. One is inorganic acid or alkaline lakes and rivers resulting naturallyfrom the inflow of acid or alkaline spas and springs. The other is artificially polluted waters by inflow of drainage from households, sewage treatment equipment and barns etc. In such environments, the existence of almost all organisms is inhibited, but diatoms are one of the group of organisms able to grow in such severe environments. Moreover, some diatom taxa occurred in large quantities in these environments. In this paper, we summarize the dominant taxon of many attached diatom assemblages occurring in severe environments from the view point of ecology.

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A slide in the Tempère & Peragallo exsiccata set (Tèmpere & Peragallo 1913), which is thought to be an isotype material for Melosira pusilla F. Meister, was examined. The form of frustules, density of striae and position and number of rimoportulae, agree with the specimen examined from Katata lagoon, Shiga Pref. supporting the results of Tuji &Houki (2004), who proposed the new combination, Aulacoseira pusilla (EMeister) Tuji et Houki. Aulacoseira subborealis (Nygaard) Denys, Muylaert & 'Crammer may also be a synonym of this taxon.

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