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Riccia (order Marchantiales) A genus of small, thalloid liverworts in which the thallus may be broad or narrow, and usually branches dichotomously (i.e. forks), often resulting in a rosette-like appearance. There are several British species. R. glauca (crystalwort) is the commonest, found on damp, cultivated soil from early autumn to late spring; the thallus is light glaucous-green. R. fluitans is variable in form and may be free-floating in ponds or ditches or may grow on mud; the free-floating form is narrower and more delicate than the terrestrial form.