It is known that the deletion of rare species from a data matrix makes ecological analysis easier. The aim of the present study is to discuss the profitability of relative abundance (ratio of individuals of a species in a sample) as a criterion of determination of rare species, although the use of relative frequency (probability of occurrence of a species in the analyzed samples) as the criterion is popular. The results indicated that deletion of less-abundant species caused as little distortion to the data analysis as deletion of less-frequent species. When the analyzed samples were varied, deletion of less-abundant species showed smaller distortion than deletion of less-frequent species. It should be noted that deletion of less-frequent species sometimes makes the data analysis impossible because most species can be deleted from small and isolated subgroups of the samples. It was also indicated that the species-compositional data analysis on the basis of similarity could be carried out with little distortion, when the most quarter, or one-third, of the observed species were analyzed by Bray-Curtis similarity index, or Pianka's alpha index, respectively.

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Teratological variations of Gomphonema parvulum Kützing were observed in a paddy drainage channel with effluent of domestic sewage. More than 40 % of the valves of G. parvulum were abnormal in winter. Abnormal frustules, including interrupted patterns of striations, irregularly directed striae and abnormal stigma, were of “Janus” type. The frequency of teratological forms was size-dependent. As valves decreased in length, the proportion of valves with abnormal striation increased. The senile change characterized by size reduction after rapid reproduction under high nutrient loading condition may cause those teratological variations.

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A new genus named Oshitea is described from the marine Nabuto Formation (early Miocene), the Boso Peninsula. It is distinctive in having a set of five silicified girdle bands that constitute as a whole a tight system of bilayered wall. In this respect Oshitea differs from any other naviculoid pennates inclusive of Pleurosigma to which it is close by the loculate wall of the valve. Oshitea consists of two new species,O. longelanceolata and miniprolongata. The affinities as well as the morphologic and functional significances of the particular girdle are briefly discussed together with another nature of the genus, a siphonic tubular raphe.

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In this study,474 epilithic diatom samples were obtained in 58 rivers in the Hokuriku District from 1974 to 1988. After examining these samples in terms of the DAIpo values, Shannon's diversity index (Di), number of taxa in each assemblage and contact-type, the followingr esults were obtained.
1) The 474 sites, where the samples were obtained, were divided into 5 groups with 5different degrees of water pollution according to the relationships, proposed by Watanabe et al. (1988), between the DAIpo values and saprobic levels. The xenosaprobic water area was aga in divided into two subgroups. All epilithic diatom assemblages in this survey were classified by the method proposed by Sumita (1990): by examining the contact-type of the first grade species (the first grade in terms of relative abundance) in each assemblage.
As a result, the diatom assemblages with their first gr a de species belonging to Type I (total contact) appeared in all water areas and increased in number as degrees of water pollution progressed, while the assemblages with their first grade species belonging to Type III (stalk contact) showed an opposite tendency. The assemblages with their first grade species belonging to Type II (point contact) appeared frequently in areas with the intermediate degrees of water pollution. The assemblages with their grade species belonging to Type II and III did not appear in the most polluted water areas (DAIpo <30).
2) The correlation between DAIpo and D i of the diatom assemblages showed the pattern of a symmetric parabola between right and left halves separated by boundary of which the DAIpcv, alues are around 50. This result corresponded fairly well with that obtained by Watanabe et al. (1988).
After examining the average values of Di of the diatom assemblages with their first grade species belonging to Type I, II and III in all water areas, the following results were obtained: in range of DAIpo 50-100, the diatom assemblages were mostly composed of mixture of Type I, II and III. Those diatom assemblages seemed to share a common tendency showing that the Di values decreased as the DAIpo values increased. But in the range of DAIpo 0-50, the diatom assemblages were mainly composed only of Type I. The Di values of those diatom assemblages seemed to decrease as the DAIp0 values of the diatom assemblages of Type I decreased.
3) Distribution of the Di values of the diatom assemblages can be seen in the f ollowingt hree groups according to the type of the first grade species.
i) The Di values of the diatom assemblages w i th their first grade species belonging to Type I were distributed in all the range of Di from mesosaprobict o polysaprobicw ater areas (DAIpo<50).
ii) The Di values of the diatom assemblages with their first grade species belonging to Type II were distributed in a comparatively narrow range of Di values betWeen 1.5 and 3.5.
iii) The diatoma ssemblages with their first grade species belongingt o Type III were the main components of most of the assemblages with high values of Di in clean water areas (DAIpo)>50).

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Hirakata Bay is a small bay in the western coast of Tokyo Bay. On 5th June 1990, a red tide which consisted of Skeletonema menzelii occurred in this bay.
The marine pelagic diatom S. menzelii was first described by Guillard et al. (1974) on its specimens collected from the western Sargasso Sea. The cell sizes of this small diatom are 3-6, μm in diameter, and it is usually living solitary or making short chains of 2-3 cells. T he transmission electron microscopy reveals that the valves have 4-8 marginal strutted processes, with a labiate process near the centre. It was the first event for this species as far as the author knows that it caused the striking propagation at a small inlet like Hirakata Bay in Japan.

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Nitzschia multistriata sp. nov. was found in some bays of southern Japan during warm seasons. The cells are usually smaller than those of N. seriata, N. pseudoseriata and N. pungens. It resembles N. actydrophila in the size, but its pseudonodulus is absent. A prominent bloom of this diatom was noted in Fukuoka Bay, in the summer of 1991.

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A new fossil Thalassiosira species, T. mizunamiensis Yanagisawa sp. nov., is described from the early Middle Miocene Oidawara Formation in Mizunami area, central Japan. This species is characterized by a subcentral fultoportula in the middle of the radius, a single rimoportula located at margin, decreasing areolae size toward the margin and distinctly ribbed fairy broad rim. T. mizunamiensis occurs from several early Middle Miocene sequences in Japan, ranging in age from the upper part of the Denticulopsis praelauta Zone (NPD 3B) to the lower part of the D. lauta Zone (NPD 4A) of Akiba's (1986) Neogene North Pacific diatom zonation. It is of potential biostratigraphic value in this stratigraphic interval.

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Middle Miocene fossil diatom assemblages were reported from the Nanatani and Minamiimogawa Formations in the Neogene Niigata sedimentary basin, central Japan. The diatoms in calcareous concretions from the upper part of the Nanatani Formation are completely pyritized and correlated to the upper part of the early Middle Miocene Denticulopsis hyalina Zone (NPD 4B) of Akiba (1986). The diatom flora in calcareous concretions from the upper part of the Minamiimogawa Formation are correlative with the latest Middle Miocene Thalassiosira yabei Zone (NPD 5C), and indicates that this formation is equivalent to the Teradomari Formation, one of the standard Middle Miocene stratigraphic units in the Niigata basin. The diatom floras from the two formations are composed of mixtures of abundant cold water species and rare warm water species, suggesting that the paleoceanographic environment of the Nanatani and Minamiimogawa Formations was influenced by cold water current, and to a lesser extent by warm water current.

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Fossil diatoms found in two diatomite samples taken from two vertically located strata about one meter above the surface of the road on the top of Katakura-toge (Katakura Pass), Mochizuki Town, Nagano Prefecture were investigated.
In these saamples,49 taxa in c luding5 unidentifiedt axa were recorded. They belong to the 18genera and 7 families. Among those taxa, two species, Cocconeis diminuta and Melosira distans are noticeable. The former alkalibiontic form is abundant,40.3 % in the upper stratum but much less in the lower stratum,6.7 %. On the other hand, the latter acidophilous form is abundant,50.8 % in the lower stratum, but less 23.6 % in the upper stratum.
Based mainly on those facts, the past sedimentary environ m ent of those strata is assumed to be a lake or marsh in which the transition occurred from acidic water caused presumably by volcanic activities to alkaline water caused by successive plant growth.

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The small freshwater shell known to be the ice age relic species, Pisidium nipponense Kuroda, distributes in Yamagata, Niigata and Nagano Prefecture in our country.
In this paper the writer has attempted to investigate the diatom t h anatocoenoses in the interior body and the digestive organ of the shell, which was collected from the small pond of Sphagnum-bog at Mt. Ogenashi located in the south-western part of Niigata Prefecture.
As a result of analysis on some diatom valves of the small tyipe, Eunotia pec t inalis var. minor, Pinnularia braunii, P. interrupta, P. subcapitata and Neidium alpinum, were detected together with Protozoa, Euglena sp., Acella sp., Paramecium sp., and Trichocera sp. of Trichocercidae.

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In this paper we tried to analyze diatoms collected from the seaweeds, piles, rope and bottom ooze of the lacustrine littoral zone at the Lake Kamo, Sado Island. The results are as follows:
(1) The analysis reveals 79 spiecies and 11 varieties of 32 genera of the diatom.
(2) The samples are dominated by the brackish water and marine species, and contained a small number of fresh water species.
(3) The diatoms in the bo t tom ooze are presumed to form the third assemblage unlike the bio-assemblage (benthonic or epiphytic brackish water and marine species). However, the assemblage are influenced by the hydrogical condition of water basin.

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