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TaxonomyDiatoms have cell walls made of silica, constructed like pill- or date-boxes, and can be preserved in sediments over many centuries. The structure of the walls can be seen after cells have been treated to remove the organic contents.Diatom species are usually identified by the shape and patterns of the silica walls. Strong, short ribs under a keel along the valve (half wall) are a diagnostic character for the genus Nitzschia, of which N. sigmoidea is the type species.The different species of Nitzschia are usually recognised by the shape and size of their valves, the density and orientation of rows of pores across their surface, and the short ribs that span the raphe, which often lies along the ridge of a lateral keel. The raphe is usually formed of a pair of longitudinal slits in the wall through which mucilage can be secreted.Diatom cells are described in two views – valve and girdle view. Cells of N. sigmoidea are more or less sigmoid in girdle view, and linear with wedge-shaped ends in valve view. Cells are about 100-500µm long (0.1-0.5mm), 8-15µm wide in valve view, and about 30µm wide in girdle view. Girdle width varies with the stage of the cell cycle. The raphe keel is slightly off-centre, with irregularly spaced ribs spanning the keel (about 5-7 in 10 µm). Parallel rows of pores cross the valve surface, 23-27 rows in 10µm.The girdle region is formed of several open bands that also contain pores.With electron microscopy more detail of the wall structure is visible.