Ranunculus rionii Lagger, Flora 31: 49. 1848; Batrachium trichophyllum (Chaix ex Villars) Bosch subsp. rionii (Lagger) C. D. K. Cook; R. flaccidus Persoon var. rionii (Lagger) Hegi; R. trichophyllus Chaix ex Villars subsp. rionii (Lagger) Soó.
Herbs annual, terrestrial or aquatic. Stems ca. 20 cm, branched, glabrous. Petiole 5--18 mm, glabrous or sheath abaxially sparsely strigose; leaf blade flabellate in outline, 0.6--1 × 0.8--1.5 cm, glabrous, 3-sect; segments 2 or 3 × sect, ultimate lobules filiform, remaining divaricate out of water. Flower ca. 0.8 cm in diam. Pedicel ca. 1.5 cm, glabrous. Receptacle subulate in fruit, puberulent. Sepals 5, elliptic, ca. 2.5 mm, glabrous. Petals 5, white with yellow base, obovate, ca. 4 × 2 mm, apex rounded. Stamens ca. 15; anthers oblong, ca. 0.5 mm. Aggregate fruit subglobose, ca. 4.5 mm in diam.; achenes obliquely narrowly obovoid, 0.8--1.2 × 0.6--0.8 mm, glabrous, with 4--7 transverse wrinkles. Persistent style ca. 0.1 mm. Fl. May.
Lakes; below 100 m. Beijing [Afghanistan, Kazakhstan, N Pakistan; S Africa, W Asia, Europe].
Cook (Mitt. Bot. Staatssamml. München 6: 142. 1966) reported that this species is distributed in Xizang and N China. However, we have seen only one specimen from Beihai Park in the city of Beijing collected by Liou Ying in 1936. The flower of this specimen has about 30 carpels but Cook (loc. cit.) stated that typical Batrachium rionii has 60–90 carpels per flower. Vegetatively this species is indistinguishable from B. trichophyllum .