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Centric diatoms appear round, discoid, or cylindrical depending on the relative length of their axes. In "valve view" they have radial or near-radial symmetry, while a few have developed some bilaterality (e.g. Auliscus and Biddulphia). In "girdle view" they are "boxy" similar to pennate diatoms. If filamentous they are always viewed "on the side" in girdle view, as the most common, Melosira spp.
At least one genus (Trigonum) is triangular in girdle view -- another form of radial symmetry, of course.