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Isoetes, commonly known as the quillworts, is a genus of lycopod. It is the only living genus in the family Isoetaceae and order Isoetales. There are currently 192 recognized species, with a cosmopolitan distribution mostly in aquatic habitats but with the individual species often scarce to rare. Species virtually identical to modern quillworts have existed since the Jurassic epoch, though the timing of the origin of modern Isoetes is subject to considerable uncertainty. The name of the genus may also be spelled Isoëtes. The diaeresis indicates that the o and the e are to be pronounced in two distinct syllables. Including this in print is optional; either spelling is correct.