Microphyllidious shrubs 1-3 m with slender, either erect or trailing defoliate trunks and virgate new branches clad in upwardly imbricate lvs but lacking axillary brachyblasts, the young stems pilosulous with mixed gray and brown hairs to 0.3-0.7 mm, the narrow, contiguous or imbricate lfts glabrous except for random cilia, dark olivaceous on both faces, low-convex and sublustrous ventrally, the capitula of bright red fls crowded into a short terminal, efoliate or only proximally foliate pseudoraceme; phyllotaxy distichous. Stipules ovate-triangular 1.5-4.5 x 1.2-2.2 mm, pubescent dorsally, weakly 7-9-nerved, deciduous. Lf-formula i/(25-)27-45; lf-stks stout 5-14 mm, the petiole including pulvinus 1-6 mm, at middle 0.6-1 mm diam, the one interpinnal segment not much longer; pinnae subequilong or the distal pair somewhat longer, the rachis of longest 3.5-6 cm, the longer interfoliolar segments 0.8-1.1 mm; lft-pulvinules 0.05-0.15 x 0.3-0.45 mm, the lfts essentially sessile against rachis; lfts abruptly decrescent at each end of rachis, otherwise subequilong, the blades linear from deltately auriculate base, obtuse or obscurely apiculate, those near mid-rachis 6.5-10.5 x 0.9-1.7 mm, 6-9.5 times as long as wide; venation seemingly of 3 slender parallel nerves extending the length of blade, the midrib displaced to divide blade ±1:2, the 2 posterior primary nerves nearly or quite as long, sometimes a fourth very short outermost one, all these simple or almost so, immersed on upper face, faintly prominulous beneath. Peduncles mostly geminate, 1-2.8 cm, bracteate at or above middle, the bract deciduous; capitula ±8—14-fld, the oblong receptacle including short terminal pedestal 3.5-5 x 2 mm; bracts 0.5-0.9 mm, tardily deciduous; fls homomorphic except the terminal one a trifle wider than the rest, the peripheral ones either staminate or bisexual, the perianth of all 4-merous glabrous; pedicels stout 0.4-0.6 x 0.6-0.8 mm; calyx of peripheral fls 1-1.5 x 1.2-1.3 mm, the depressed-deltate teeth 0.20.3 mm; corolla 4—5.2 mm, the ovate lobes 1.4—2.3 mm; androecium 6-7-merous ±4-5 cm, the tube 1.4—2 mm, the stemonozone <1 mm; ovary sessile glabrous. Pods in profile 6-11 x 0.8-0.9 cm, the sutural keels in dorsal view ±3-4 mm wide, the recessed valves sinuously obliquely venulose, the whole becoming fuscous or nigrescent, minutely pilosulous overall with erect hairs; seeds (not seen fully ripe) light brown, smooth: pleurogram 0.

In brush thickets on and around arenitic outcrops, 1000-1100 m, localized on slopes of the Pakaraima Mts. in e. Bolívar (Gran Sabana), Venezuela, and immediately adj. Guyana (sources of Mazaruni R.). — Map 49. — Fl. II, VI-IX, perhaps at intervals throughout the year.

Calliandra rigida has crowded, imbricately upswept leaves that recall the Bahian C. calycina, but the parallel leaflet-venation of compatriot Guayanan species. It is readily distinguished from all these (ser. Tsugoideae) by the glabrous perianth, and further from its close neighbor C. pakaraimensis by smaller leaves and leaflets, much smaller flowers, and 6-7-merous androecium.