Moderate to robust, clustering, spiny, hapaxanthic, polygamous, acaulescent or tree palms. Stem subterranean, erect, or borne on robust stilt roots at heights up to 3 m above the ground, branching sympodially by basal (?axillary) suckers, internodes very short to moderately elongate, usually covered by rotting leaf sheaths, becoming exposed in tree species and sometimes bearing short spine-like adventitious roots; cortex very hard, pith soft with abundant starch deposition before flowering. Leaves pinnate, spirally arranged or markedly 3-ranked; sheath usually splitting opposite the petiole, unarmed at the very base, bearing black, flattened spines distally, sometimes also bearing scales and branched hairs, sheath margin ligule-like distally; petiole well developed, adaxially deeply channelled in proximal portion, distally ±rounded in section, the abaxial surface sparsely to densely armed with black, flattened spines, scattered, paired or in longitudinal rows, scales and hairs usually abundant between spines, sometimes absent; rachis armed as the petiole but more sparsely; leaflets single-fold, numerous, linear to lanceolate, entire, regularly arranged or grouped and fanned within the group to give the leaf a plumose appearance, frequently bearing bristles along the main veins or the margins, and irregular bands of caducous indumentum, midribs prominent, transverse veinlets moderately conspicuous. Inflorescence erect, composed of branches equivalent to the axillary inflorescences of pleonanthic palms, each branched to the fourth-order, and subtended by leaves with much reduced blades or by tubular, apiculate, spiny or unarmed, dull brown, longitudinally imbricate bracts; branches of all orders bearing a tubular, 2-keeled prophyll, and terminating in a cupule of dull brown, longitudinally striate, spirally arranged, or more often subdistichous, tightly sheathing bracts enclosing a flower pair; cupule bracts 11–13, rarely 1 or 2 more, the 1–3 most proximal and 1–3 most distal each with an abortive axillary bud, the rest empty, the most proximal up to 5 tubular, the rest open; flower pair consisting of a large staminate and lateral to it a large hermaphroditic flower, the staminate appearing first, then pushed out of the cupule by the developing hermaphroditic. Staminate flowers borne on a short flattened pedicel, the flower base compressed on one side by the developing hermaphroditic bud; calyx tubular, coriaceous, striate, dull brown, with 3 short, pointed lobes; corolla tubular in the basal 1/4 to 1/3, distally with 3 narrow, elongate, woody, valvate lobes terminating in hard, sharp, spine-like tips; stamens 20–70 borne just above the mouth of the corolla tube, filaments short, erect, anthers narrow, elongate, basifixed, dull yellow to purple, latrorse to introrse, deciduous after anthesis; pistillode minute. Pollen ellipsoidal, bi-symmetric; aperture distal, brevi- or extended sulcate; ectexine tectate, coarsely perforate, or foveolate, aperture margin similar; infratectum columellate; longest axis 41–73 µm; post-meiotic tetrads tetragonal [2/6]. Hermaphroditic flowers protandrous, sessile, bearing a 2-keeled, coriaceous, tubular prophyll, similar to the cupule bracts, the whole flower very similar in size and shape to the staminate except for the apex, flattened on one side by pressure of the staminate flower in bud; calyx, corolla and androecium like those of the staminate; gynoecium tricarpellate, triovulate, ovary columnar, faintly 3-angled, covered in vertical rows of minute reflexed scales, stigma conical to pyramidal with 3 glandular angles, ovules basally attached, anatropous. Fruit ovoid, beaked, stigmatic remains apical, cupule bracts, calyx, and usually the corolla persisting; epicarp covered in irregular vertical rows of very small reflexed, fringed scales, mesocarp somewhat corky at maturity, traversed by longitudinal fibre bundles, endocarp developing from a layer external to the locule wall, dark brown to blackish, very hard and thick, sometimes linked to the fibres of the mesocarp, with 3 + 3, or 3 + 3 + 6 flanges penetrating into the fruit cavity, forming symmetrical, incomplete partitions. Seed basally attached, single, filling the fruit cavity and closely adhering to the endocarp and thus indented by the incomplete partitions, seed coat thin, dry, endosperm homogeneous; embryo basal. Germination remote-ligular; eophyll pinnate. Cytology not studied.
Stemless or short-stemmed often viciously spiny hapaxanthic pinnate-leaved palms of the Malay Peninsula and Borneo; flowers very large, with spine-tipped woody petals, borne in pairs of a staminate and a hermaphroditic flower in a cupule of overlapping leathery bracts. The fruit is most unusual in having myriads of minute scales and a thick endocarp.
Leaf (Tomlinson 1961) distinguished from those of other calamoid genera by short foliar sclereids in some species, perhaps supporting the rather isolated position of the genus. Floral development indicates the apparently terminal flower pair to be lateral and to consist of a typical dyad. The gynoecium consists of three carpels with ventral sutures open, and ovules initiated directly on the large apex of the floral axis (Uhl and Dransfield 1984). The flowers are among the largest in the palms, and also unusual is the development of first the staminate and later the hermaphroditic flowers, so that at any one time the plant seems to bear large solitary flowers.
In Borneo, E. insignis, E. utilis and E. minor usually seem to be associated with poor soils that have abundant humus. They are particularly conspicuous on scarp faces or sharp ridgetops. Eugeissona minor and E. insignis are also found in low-lying ‘kerangas’ (heath) forest. Eugeissona ambigua, known only from the type, was collected on a slightly raised heathy area in the Kapuas lake district of West Kalimantan. In the Malay Peninsula, E. brachystachys is associated with richer soils on hillslopes, particularly where some flushing takes place, and E. tristis is found in a wide range of forest types, from swamp margins to hilltops but grows in greatest abundance on ridgetops up to 1000 m altitude. All but E. ambigua, about which very little is known, grow in large colonies.
All species have a wide range of local uses. Sago from the stems of E. utilis forms the staple of the nomadic Penan people of Borneo and E. insignis can be used similarly. Leaves of all species may provide thatch. Petioles are used in the manufacture of blinds, blowpipe darts and toys, and the pith of the petioles for the occlusions on blowpipe darts. The young endosperm is edible and even the pollen has been eaten. Stilt roots of E. minor make excellent walking sticks. Eugeissona tristis has become a serious pest of Hill Dipterocarp forest in Malaya, where it dominates the undergrowth after logging, thereby preventing regeneration of commercially important timber trees.