A short discussion of the taxonomy and morphology of the Dutch species of Riccia the methods of studying them, followed by a key and descriptions, are given. It is stressed that fresh specimens ought to be examined. Especially delimitation of the species closely related to Riccia fluitans L. emend K.M. (1940) is rather difficult, owing to the fact that the waterforms of these species have often very much the same appearance and structure; the terrestrial forms only show distinct differences. The species concerned are: Riccia canaliculata . Hoffm., with terrestrial form with a compact thallus structure, spores 75—100/t diam. Riccia huebeneriana Lindenb., thallus with a pitted appearance, spores 50—65/*; R. rhenana Lorb. terrestrial form, broad thallus (up to 2 mm. broad at the outer end), short branches, widely divergent. The name Riccia fluitans L. emend K.M. is retained for the plant which has a terrestrial form that differs rather slightly from the waterform both in branching and structure. This division, given by K. Muller (1940,1941) of Riccia fluitans L. sensu lato, is based upon culture experiments performed by K. Muller and Lorbeer and partly repeated by the author in the greenhouses of the Hortus Botanicus, Amsterdam. It shows the true position of the much discussed (for example by Evans, 1921") plant, named „var. decipiens” of. Ricciocarpus natans by Schiffner (1939) and Riccia huebeneriana var. natans by Torka [Hedwigia 50 (1911), 205], both synonymous with R. rhenana Lorb. According to cytological research by Lorbeer (1934, Jahrb. Wiss. Bot. 80), partly reported by K. Muller (1940, 1941), the chromosome number of R. rhenana Lorb. is 16 and of R. fluitans L. and R. canaliculata Hoffm. it is 8. R. duplex Lorb. (ex. K. Muller, 1940, p. 300) has n --= 16 too, but this form has not yet been found in Holland. So it is probable that polyploidy plays a role here. The facts known of the distribution and ecology of these forms don’t suggest that they are geographical subspecies or ecotypes, of R. fluitans sensu lato. Taking all known facts into consideration the author felt himself compelled to follow K. Muller by calling them species. Of R. rhenana Lorb. only a few collections have been recorded for Holland. These are the first outside Germany. It is indicated however that Miss Carter’s (1935) “broad form” of R. fluitans L. sensu lato, from California, possibly belongs to R. rhenana. Another very critical form was recently found in Holland and named Riccia subbifurca Warnst. ex Crozals. (Rev. Bryol. 1903) The relation of this very rare species in Europe (Paris, France and Dalmatia) with R. baumgartneri Schiffner (Austria) and R. nigrella D.C. (atlantic-mediterranean) is discussed. The greatest difference from R. nigrella lies in the diameter of the spores (60— 8o(« in nigrella, against 90—iiom in subbifurca). It shows a similar border of dark ventral scales along the small glaucous green thallus in dry state. Cultivation experiments are also needed here to settle the taxonomy. Riccia oelandica Jensen belongs to the same “Formenkreis” as R. subbifurca. The other Riccia-species found in the Netherlands are Riccia crystallina L. growing especially in the river district, R. beyrichiana Hampe, — bifurca Hoffm., — sorocarpa Bisch. and — glauca L., growing on the bare soil at sides of ditches, banks etc., especially were the soil is rather rich in lime. Riccia ciliata Hoffm, is a doubtful indigene. In all ten out of the thirty one European species have been found in this country.