The glossy, light green, concave leaves, with lax, elongate upper leaf cells and generally strongly differentiated basal cells will separate G. acuminatum from other species of the genus. Although the leaf is generally concave, above the base, when cut in cross-section the entire lamina is broadly recurved. This is apparently due to a slight thickening of the dorsal cell facies.
Plants small, forming loose turfs, light green to yellowish green, glossy; terricolous. Stems (2-)5-10 mm long, sparsely radiculose below; in section cortical cells in 2-4 rows. Leaves erect-appressed, little altered dry, concave; ovate to ovate-acuminate or ovate-lanceolate, 0.5-0.8 mm long; apex acute to acuminate; base flat, not concave; margins erect, entire. Costa short-excurrent, mucro 0.1-0.2 mm long; in section subround, ventral cells 2, large, thin-walled, dorsal substereid band 2 cells thick, dorsal surface cells undifferentiated. Upper laminal cells mostly 6-10: 1, fusiform to linear-rhomboidal, ± sinuolate, (50-)75-150 µm long, 12-18 µm wide, dorsal surface weakly thickened; basal cells generally distinct,quadrate to short-rectangular, 1-2: 1, thin-walled. Perichaetial leaves triangular, ± 1 mm long. Seta (6-)10-15(-22) mm long, yellowish; capsule suberect to inclined or almost horizontal, clavate to cylindrical-pyriform or occasionally smaller and pyriform, 2.5-3.5 mm long, yellowish, neck sulcate, to 1 mm long; exothecial cells subrectangular, walls ± wavy, 1-2 rows of transversely rectangular cells at mouth; peristome teeth 200-300 µm long, endostome with basal membrane 175-200 µm high, segments rudimentary; spores round, 16-18 µm, green, weakly papillose.
Plants in loose turfs, yellowish green, glossy. Stems 2-10 mm long. Leaves erect-appressed, concave, ovate to ovate-acuminate, 0.5-0.8 mm long; margins entire; costa short-excurrent. Upper laminal cells fusiform to linear-rhomboidal; basal cells distinct, quadrate to short-rectangular. Capsule inclined to horizontal, clavate to cylindrical-pyriform; endostome segments rudimentary. Recognised by the light green, glossy plants with concave leaves, lax, elongate upper laminal cells and strongly differentiated basal cells.
Brachymenium acuminatum has been reported from eastern and southern Africa, South America, India, southeast Asia and Australia. In southern Africa the species is found in the northern, central, southern and eastern Transvaal, Swaziland, Natal and Lesotho. A few specimens have also been collected in the northwestern and southwestern Cape, eastern Orange Free State, central South West Africa/Namibia and Botswana.