Stems prostrate, tightly attached to substratum throughout, cylindrical to flattened in cross-section, (0.7) 2 (3.5) mm wide, repeatedly dichotomously or subdichotomously branched. Leaves simple and entire (uncommon), or palmately, pinnately or irregularly lobed, (1.3) 8.6 (20) × (0.6) 1.5 (7) mm, (0) 5 (7) lobed, lobes (0.2) 1.2 (4.4) mm × (0.2) 0.4 (1.2) mm, (0) 5 (7) veined. Spathella (2.6) 3 (3.3) × (1.2) 1.4 (1.7) mm, rupturing apically or subapically into (3) 6 (11) linear to triangular segments. Flower pedicel (0.2) 0.3 (0.4) mm prior to anthesis. Tepals 2 (occasionally 3), hair-like (easily overlooked), one on either side of fused stamen filaments, (0.3) 0.5 (0.6) mm, apex acute. Stamen filaments prior to anthesis (1.2) 1.3 (1.8) × (0.3) 0.4 (0.5) mm, elongating to (0.9) 2.9 (3.8) mm during anthesis; anthers (0.8) 1 (1.1) × (0.4) 0.5 (0.5) mm. Pollen polar diameter (16.2) 17.4 (20) μm, equatorial diameter (17.5) 19.4 (20) μm, sexine 0.5 μm, nexine 0.6 μm. Ovary in preanthesis flowers (1.2) 1.3 (1.4) × (0.8) 1 (1.2) mm; in post anthesis flowers (1) 1.5 (1.7) × (0.8) 1.1 (1.3) mm. Ovules (63) 93 (124) per ovary. Stigmas (0.5) 0.7 (0.8) mm prior to anthesis, elongating to (0.5) 1.1 (2) mm during anthesis. Capsules (0.9) 1.2 (2) × (0.6) 0.8 (1.1) mm; upper (dorsal) valve (0.8) 1.1 (1.8) × (0.5) 0.7 (1.1) mm, (3) 5 (7) ribbed (nonsuture), papillate; lower (ventral) valve (0.8) 1.2 (1.8) × (0.2) 0.8 (1) mm, (5) 7 (9) ribbed (nonsuture). Seeds (0.11) 0.21 (0.35) × (0.07) 0.15 (0.9) mm; (0) 23 (61) per capsule. Discussion: Dried plants on exposed outcrops often become whitened. Van Royen (1954, Plate 4) illustrated the leaves of C. multipartita as simple. As there are no leaves on the type specimen, the only specimen of the species listed by him, his interpretation is questionable. Phenology: Most specimens studied were past flowering. Flowering evidently June-July. Distribution and Habitat: Common in northern Mato Grosso and southern Pará, occasional in northern Goiás, Brazil (Fig. 9). Abundant in some rivers in Mato Grosso, often forming dense carpet-like growth on large rock outcrops and sometimes stems of rheophytic shrubs (e.g. Myrtaceae). Castelnavia multipartita f. multipartita can occur with C. fluitans and C. princeps. Representative Specimens Examined: BRAZIL. Goiás: Jardim Paulista, São Patrício River, 49° 24’51” W, 15° 0°11” S, 5 Jun 2005, Philbrick et al. 5820 (MEXU, R, WCSU). Mato Grosso: Alta Floresta, Teles Pires River, 55° 59’43.3” W, 9° 38’22.2” S, 16 Sep 2007, Bove & Philbrick 1855 (R, WCSU); Paranaíta, Teles Pires River, 56° 50’31.72 W, 9° 14’33.62 S, 19 Sep 2007, Bove & Philbrick 1875, 1877 (CONN, R, WCSU); Aripuanã, Aripuanã River, 56° 26’58.8” W, 10° 9’58” S, 23 Sep 2007, Bove & Philbrick 1892 (CONN, R, WCSU); Colniza (Panelas), Roosevelt River, 60° 44’25.8” W, 9° 9’; 49.2” S, 25 Sep 2007, Bove & Philbrick 1904 (CONN, R, WCSU); Idem, Aripuanã River, 59° 19’27.8” W, 9° 21’5.4” S, 26 Sep 2007, Bove & Philbrick 1914 (CONN, R, WCSU); Idem, Guariba River, 60° 17’47” W, 9° 14’26.9” S, 26 Sep 2007, Bove & Philbrick 1911 (R, WCSU). Pará: Jacareacanga, São Benedito River, 56° 35’7.3” W, 9° 3’ 11.5” S, 18 Sep 2007, Bove & Philbrick 1861 (CONN, R, WCSU); Idem, Serra dos Carajás, Itacaiunus River, 14 Jun 1982, Sperling et al. 6174 (NY, WCSU).