Plants aquatic, upright, often dying back to rhizome in winter [dry season], 1--3.5 m. Rhizome not evidently enlarged or specialized for starch storage. Stems unbranched below inflorescence; leaves all basal or rarely with single cauline leaf borne above elongate (0.7--2.5 m) internode. Leaves homotropic; sheath not auriculate, spongy, containing prominent air spaces; blade plain green, ovate to elliptic. Inflorescences branched, branches short and upright to elongate and arching; rachis internodes conspicuously zigzagged; bracts deciduous, both bracts and prophylls falling with flower if fruit not set, leaving proximal portion of rachis bare, each bract subtending 1 flower pair, herbaceous to leathery; prophylls not evidently keeled, membranous; secondary bracts absent; bracteoles absent. Flowers self-fertilizing [outcrossing], pale to dark purple (corolla and staminodes); sepals persistent in fruit, 0.5--3 mm, membranous; corolla tube 1--6 mm, corolla lobes subequal to strongly unequal; outer staminode 1, petal-like, showy; callose staminode mainly fleshy, narrow apical rim petal-like; cucullate staminode with 2 appendages, subterminal, finger-like; stylar movement helical when tripped; styles with 1 appendage, elongate, straplike. Fruits capsules, 1-seeded, nearly globose to ellipsoid, pericarp thin, indehiscent. Seeds dark brown, nearly globose or ellipsoid, smooth; perisperm canals 2, curved; aril reduced. x = 6. Thalia is not closely related to any of the other Neotropical genera (L. Andersson 1981), and it is the only genus of Marantaceae that is strictly aquatic. The cucullate staminode bearing two appendages, the helical stylar movement, and the appendaged style are unique within the family. Also unique is the prevalence of various air chambers in the leaf sheath and petiole, a specialization for the aquatic habitat, which gives the sheath its distinctive spongy texture. Dispersal by water is probably the most common means of dispersal: a gas-filled space between the seed and the fruit wall in Thalia makes the fruit buoyant (C. J. Grootjen 1983). Fruits of T. geniculata have also been reported to be eaten by ducks (W. L. McAtee 1915). Carpenter bees (Xylocopa spp.) are reported to pollinate T. geniculata in Costa Rica ( T. Hartgerink et. al. 1978) and in cultivation in Asia (R. Classen-Backhoff 1991). The widely varying chromosome numbers reported for species of Thalia probably represent errors or misinterpretations. Chromosome counts in Marantaceae are difficult, and few taxa have been adequately studied.