Tall, usually clustered, spiny, pleonanthic, monoecious palms. Stems erect, often very tall, becoming bare, ringed with leaf scars, frequently armed with scattered or dense, robust spines, the spines tending to erode with age, in one species (Oncosperma fasciculatum) the stems sometimes branching aerially. Leaves pinnate, neatly abscising; leaf sheaths tubular, closely sheathing, forming a well-defined crownshaft, bearing abundant tomentum and spines of varying lengths, the distal margins tending to tatter irregularly; petiole usually robust, adaxially concave or flattened, abaxially rounded, usually densely armed with short to long spines and indumentum; rachis adaxially angled, rounded abaxially; marginal reins sometimes well developed and persisting as pendulous strips at the base of the rachis, leaflets very numerous, single-fold, acute or acuminate, regularly arranged, pendulous or not, or distinctly grouped and held in different planes (O. fasciculatum), lacking spines, bearing bands of caducous scales adaxially, abaxially with ramenta along the main vein, and sometimes with dot-like scales. Inflorescences solitary, infrafoliar, branching to 2 orders near the base, to 1 order distally, protandrous, erect in bud; peduncle short, broad, winged at the base, horizontal at anthesis, sparsely to densely armed with straight spines; prophyll inserted just above the base of the peduncle, robust, tubular, flattened, 2-keeled, briefly beaked, completely enclosing the inflorescence until anthesis, coriaceous to ± woody, densely indumentose, very sparsely to very densely covered in short or long, straight or twisted spines; peduncular bract 1, inserted just above and similar to the prophyll but only slightly 2-keeled and usually more sparsely armed; rachis longer than the peduncle, bearing few to numerous, pendulous first-order branches; rachis bracts very inconspicuous; rachillae long, slender, rather robust, flexuous, pendulous, glabrous, bearing spirally arranged, low triangular bracts each subtending a triad, or in the distal portion of the rachilla, subtending solitary or paired staminate flowers. Staminate flowers asymmetrical, sessile or sometimes slightly stalked at the base; sepals 3, ± acute, keeled, imbricate, basally briefly connate or ± distinct; petals 3, acute or acuminate, valvate, much longer than the sepals, distinct or briefly connate at the base; stamens shorter than the petals, 6–9 (?12 recorded but not found by us), briefly epipetalous or ± free, the filaments short, thick, sometimes connate at the base in a very short ring, anthers ± medifixed, basally sagittate, latrorse; pistillode shorter or longer than the stamens, deeply trifid. Pollen ellipsoidal symmetric; aperture a distal sulcus; ectexine tectate, coarsely perforate-finely reticulate with supra-tectate, angular, striate, block-like clavae, aperture margin similar; infratectum columellate; longest axis 27–43 µm [4/5]. Pistillate flowers ± globular; sepals 3, distinct, rounded, imbricate; petals 3, distinct, rounded, imbricate at the base, the tips valvate; staminodes 6 (?always), very small, tooth-like; gynoecium unilocular, uniovulate, ± globose or ovoid, the stigmas apical, scarcely prominent, ovule form unknown. Fruit spherical, dark blue-black, with lateral or subapical stigmatic remains; epicarp smooth or pebbled, mesocarp thinly fleshy, without fibres, endocarp thin, crustaceous, closely adherent to the seed, with a round basal operculum. Seed attached by an elongate lateral hilum, raphe branches anastomosing, partially embedded in the endosperm, endosperm deeply ruminate; embryo subbasal. Germination adjacent ligular; eophyll bifid. Cytology: 2n = 32 (± 2–4).
Oncosperma tigillarium is characteristic of the landward fringe of mangrove forest and is also rarely found on poor sandy soils inland; O. horridum, O. platyphyllum and O. gracilipes are characteristic of hilly sites inland and O. fasciculatum occurs on steep slopes at altitudes of 300–1000 m above sea level. Species of Oncosperma frequently occur gregariously and are very conspicuous; their heavy spiny litter may play an important role in preventing regeneration of dicotyledonous trees below the palms. Little is known of the natural history of these common palms (see House 1984).
The cabbage of most species is excellent and widely collected. The trunks of O. tigillarium are extremely durable and have been used (with spines removed) as telegraph poles and split as flooring. Sheaths are sometimes used as buckets. Despite their spines, all species are very elegant ornamentals.