Medium-sized to very large trees, often with prominent, thick, rounded, tall straight buttresses. Crown comparatively small, appearing irregularly hemi-spherical and oblong, rather diffuse, with a few large twisted branches ascending from the bole apex; branchlets not crowded towards the apices. Bark surface rather shallowly irregular-section fissured, the fissures separating ± flat flaking ridges; dotted with warty lenticels. Young parts at first ± densely lepidote with emarginate peltate hairs; lamina undersurface persistently so. Twigs ribbed. Stipules relatively large, narrow, fugaceous. Leaves oblong to obovate, base usually obtuse, apex shortly acuminate; nerves curving round and anastomosing at the apices, thus forming a looped intramarginal nerve (cf Cotylelobium); tertiary nerves densely or remotely scalariform; petiole distinctly geniculate. Inflorescence long, lax, pendent, densely tomentose; bracteoles small, linear, caducous. Flower bud distinctly pedicellate, distichous. Calyx +-imbricate, rarely valvate (A. laevis); with 2 obtuse outer lobes slightly more thickened than 3 inner acute lobes, united at base in an indistinct (in flower) tube round the partially inferior ovary. Petals oblong-linear, falling separately. Stamens 15-65 in 3 verticils or irregular, the outer somewhat shorter than the inner, glabrous; filaments rather short, slender, filiform, tapering, connate at base; anthers with 4 pollen sacs, the inner 2 shorter than the outer 2, latrorse; appendage to connective short or long. Ovary semi-inferior, with a distinct stylopodium; style long or short, obscurely trifid; stigma indistinct. Fruit calyx valvate with an ellipsoid basal tube almost entirely enclosing and adnate to the nut, with 2 long narrowly spatulate obtuse untwisted 3-nerved lobes, and 3 acuminate short lobes. Nut ± globose, enclosed but for the apex in the valvate tube, with distinct stylopodium. Germination epigeal, pericarp splitting open irregularly apically; cotyledons unequal; first leaves paired, with interpetiolar stipules, or in a whorl of 4, without stipules; followed by spiral leaves as other genera.
Lowland forests, rarely above 1000 m. Scattered in mixed Dipterocarp, Mixed Swamp and Heath forests of the humid zone but becoming gregarious in N. Malaya, Indochina and parts of New Guinea.
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