Small, moderate or tall, solitary or clustered, unarmed, pleonanthic, monoecious palms. Stems erect, slender to robust, bare, conspicuously ringed with leaf scars. Leaves entire-bifid or pinnate, neatly abscising; sheaths elongate, forming a well-defined crownshaft, usually densely scaly or tomentose, and/or waxy, a ligule-like prolongation sometimes present opposite or at the base of the petiole; petiole short to long, adaxially channelled, abaxially rounded, usually conspicuously scaly; rachis adaxially channelled or angled near the base, distally angled, abaxially rounded, usually scaly as the petiole; leaflets regularly arranged, or grouped, pendulous or horizontal or ascending, straight or curved, single-fold or several-fold, the terminal pair usually broad, several-fold, the rest parallel sided or somewhat wedge-shaped, apically acute, bifid or conspicuously praemorse, adaxial and abaxial surfaces bearing scattered minute scales, abaxially sometimes with scattered ramenta along the main veins, sometimes also with bands of deciduous chaffy scales along major ribs, transverse veinlets conspicuous or obscure. Inflorescences infrafoliar, branching to 1–3 orders or rarely spicate, usually horsetail-like, protandrous or protogynous; peduncle short, winged at the base, sometimes becoming swollen; prophyll compressed, entirely enclosing the inflorescence in bud, 2-keeled, with a conspicuous apical beak, thin, papery when dry, glabrous or scaly, soon drying on exposure, splitting longitudinally on the abaxial face and abscising together with the peduncular bract; peduncular bract 1 rarely 2, similar to and entirely enclosed by the prophyll, tubular, enclosing the inflorescence in bud; subsequent bracts inconspicuous; rachis (where present) longer or shorter than the peduncle, bearing inconspicuous rachis bracts subtending few to many crowded, ± spirally arranged first-order branches, the proximal bearing a few branches or all unbranched; rachillae elongate, usually ± straight or curved, of ± equal length, tending to curve downwards, bearing throughout their length spirally arranged or opposite and decussate pairs of triads of cream-coloured or pinkish-tinged flowers, except at the very tip where bearing solitary or paired staminate flowers; rachilla bracts very inconspicuous, low, ± rounded. Staminate flowers fleshy, asymmetrical; calyx sessile or with a short stalk-like base, sepals 3, short, triangular, ± distinct or joined into a cup for ca. 1/2 their length; petals 3, fleshy, distinct, except at the very base, valvate except in Hydriastele palauensis where margins not meeting triangular, asymmetric; aperture a distal sulcus or trichotomosulcus; in bud, 4–5 times as long as the calyx, narrow, triangular, 1 usually larger ectexine tectate, coarsely perforate, foveolate, coarsely perforate-than the other 2; stamens 6–24, epipetalous, filaments very short, fleshy, rugulate or rarely scabrate verrucate, aperture margin similar; longest variously epipetalous and connate, anthers elongate, erect, basifixed, axis ranging from 33–70 µm; post-meiotic tetrads tetragonal or latrorse, connective sometimes prolonged into a short point; pistillode tetrahedral [22/47]. Pistillate flowers globose or ± conical in bud, smaller absent. Pollen ellipsoidal, bi-symmetric; aperture a distal sulcus, brevi-, ± than the staminate; sepals 3, distinct, rounded or triangular, broadly same length as long axis or, frequently, extended; ectexine semi-tectate imbricate or connate in a ring with 3 low triangular lobes; petals 3, and coarsely (rarely finely) reticulate, muri of reticulum sometimes distinct or connate, not more than to at least twice as long as the sepals, perforate, aperture margins similar; or pollen ellipsoidal or oblate-rounded or triangular, basally broadly imbricate or connate in a ring, apically rounded except for very small triangular valvate tips or with conspicuous triangular valvate tips, closely appressed in bud, the tips persisting or eroding into fibres in fruit; staminodes 3(–6), tooth-like, minute; gynoecium ± globose or ovoid, unilocular, uniovulate, stigmas 3, low, sessile or fleshy, reflexed, ovule laterally attached near apex of locule, hemianatropus (?always). Fruit globose to narrowly ellipsoidal, straight or curved, bright red to purplish-black, sometimes drying ridged, sometimes briefly beaked, stigmatic remains apical, perianth whorls persistent, the petal tips sometimes reflexed or appressed to the fruit; epicarp smooth or slightly pebbled, mesocarp thin, with abundant tannin cells, and longitudinal fibre bundles, endocarp thin, crustose or obsolescent. Seed ovoid or globose, laterally or basally attached with elongate or rounded hilum, raphe branches sparse, anastomosing, endosperm homogeneous or shallowly to deeply ruminate; embryo basal. Germination adjacent-ligular; eophyll bifid with entire or minutely to strongly praemorse tips. Cytology: 2n = 32.
Lowland to upland tropical rain forest. One species, Hydriastele rheophytica, occurs as a rheophyte in western New Guinea, and has very slender leaflets (Dowe and Ferrero 2000). Several species are recorded from limestone and others from ultramafic rock. Pollination has been studied by Essig (1973) who showed that curculionid beetles are probably the pollinators in H. microspadix.