Associated with the ordinary type of frustule, the girdleless frustules, prevalent in the material from the marine sediments of the Miocene Nabuto Formation distributed in the Boso Peninsula, represent another morphotype in which one valve is identical with that of the ordinary type while the other differs by having internal depressions at the valve margin. This type of frustule is most perplexing in that there is no space inside them, other than the region around the valve mantle where the depressions lie, due to the tight adhesion of valve to valve. It is concluded that the morphotype must be involved in some process to surmount the dormant condition of the organism that had probably lived within the ordinary frustule.
Two new genera, both of which concern this unique morphotype, are introduced from operative evidence they share and are delineated by the foamy-walled valves, the uniformly produced rimoportules on the valve mantle and the lack of a pseudonodulus. They are Spumorbis, which includes three new species and has discoidal valves with areolae radiating in fascicular patterns, and Araniscus, which is further separated from Spumorbis by an intimate linkage of the elliptic-contoured valve to the cobwebby array of areolae. Within Araniscus three are described as new and one known Coscinodiscus lewisianus is newly combined. Both the genera are close to Actinocyclus.

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In Japan, two species Arachnoidiscus ornatus and A. ehrenbergii have been reported as the recent species in this genus. They are well known as the epiphyte, and often cover the whole surfaces of seaweeds. Materials collected from 10 localities in Japanese coast show no difference except for the diameter of valve. It is concluded that the Japanese recent entity of Arachnoidiscus can be named under A. ornatus with a synonym of A. ehrenbergii.

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Specimens collected from Sayama-ike (Sayama Pond: Osakasayama, Osaka, Japan; sediment core) and Naktong River (Kyongsang-Nando, Korea; plankton) were examined in detail by using scanning electron microscope, and identified as Stephanodiscus vestibulis. The species is characterized by a vestibule which is a small space at the outer opening of marginal fultoportula. In addition to the arch-like opening of vestibule reported in the original description, fan-shaped, and very narrow slit openings were observed. The number of cowlings of a central fultoportula is three according to the original description, however, some specimens possessed two cowlings were observed. In light microscopy, this species can be distinguished from similar species Stephanodiscus minutullus and S. delicatus by a somewhat large spot on the interfascicle near the valve margin, and not clearly observed marginal fultoportulae. It was recorded from eutrophic freshwaters of western North America, Japan and Korea.

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Necessary sample sizes, and adequate estimators, to estimate diversity indices are discussed. Three samples of epilithic diatom communities collected from different rivers were used to decide the necessary sample sizes. From each sample,14400 frustules were counted. Using these samples, three indices of diversity were examined: Shannon-Wiener's index, Simpson's index, and Itow-Morisita's index, Some unbiased estimators of each index were also examined. These unbiased estimators usually reduced the bias of the standard estimators. However, the variance of unbiased estimators was always larger than that of the standard estimators. Judging from the mean-square error (MSE), the two unbiased estimators (Morisita's and jackknife estimators) of Shannon-Wiener's index were better than the standard estimates. In contrast, the standard estimator of Simpson's or Itow-Morisita's index was better than the unbiased estimators. When the sample size was 400, the square-root of MSE was usually <5% of Shannon-Wiener's index, and it was always <10% of Simpson's- or Itow-Morisita's index.

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Diatom assemblage in the sediment of Lake Yamanaka was examined. A total of 221 taxa among 37 genera were recorded. All taxa, which occurred abundantly, were planktonic species, i. e. Aulacosira granulata, Cyclostephanos dubius, Asterionella formosa, Cyclotella radiosa, Tabellaria fenestrata, Fragilaria capucina. DAIpo value calculated with the relative frequencies of the taxon observed was 56.5.

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On February 1998, our research team conducted a study on diatom vegetations at eight inland waters in Neco Harbour, Antarctic Peninsula (64° 51' S 62° 30' W) and Cuvervill Island (64° 41' S 62° 35' W). The diatom compositions were very simple and the most dominant species contributed the most part of the diatom assemblages. The dominant species was Navicula muticopsis Van Heurck (Luticola muticopsis (Van Heurck)Mann) which has a bipolar distribution. Some marine species appeared with a low frequency in both locations. The contamination with marine species probably resulted from the introduction of air bome salt to the inland waters and the transportation by many penguins inhabiting the vicinity. The standing crop (expressed by cell number) at one sampling site was almost the same as in oligotrophic lakes, although the water was eutrophic.

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Vol. 13 No. 0 p. 2

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Vol. 13 (1997) No. 0 p. 96

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Vol. 13 (1997) No. 0 p. 174-175

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Vol. 13 (1997) No. 0 p. 263

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