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Abstract
This study deals with the taxonomy of the marine diatom Roperia tesselata (Roper) Grunow: size, valve shape, areolation, rimoportula, valve margin, and geographical distribution are all considered. Waters in which Roperia tesselata occurs are divided according to water temperature and geographical distribution patterns; one is a warm water, western hemisphere habitat and the other a cold water, northern Pacific Ocean habitat. Comparisons between specimens from the two regions were made on, (1) size—cold water cells are larger than those from warm water; (2) rimoportula openings—the cold water cells have simple external openings, while those of the cells from warm waters have horizontal slits with a slightly widened centre; (3) the cells from cold water have decussate areolation, and those from the warm water have a linear pattern in the centre and a radial pattern at the margin. The difference in the external openings of the rimoportulae is reflected also in the outline of the valve as seen in the light microscope: the outline appears smooth in the cold water forms, but in the warm water forms it is conspicuously undulate on the side distant from the pseudonodulus.