Material seen. — SARAWAK. 1st Div.: SE. end of limestone outcrop S. of Bau (leg. Wilford, UF I94835/>10; do. 194836/8; do. I94845/>10); Kpg. Piching, near Serian (BMNH/1, see above; NMW/7); G. Selabor, W. of Kpg. Lobang Batu, 12.5 km S. of Tebakang (leg. Wilford, UF I94837/>10; do. I94838/>10; do. 194839/>10; do. 194840/>l0; do. 194841/>10; do. 194842/>10; do. 194843/>10; do. 194844/>I0; V 2093/>10).

Spire conical with slightly to distinctly concave sides. Apex not oblique. Whorls 5 1/4-5 7/8; top whorls convex; next whorls slightly convex; last whorl almost sharply angular at the periphery, almost flat to slightly concave above, almost flat below. Constriction with a parietalis, a transverse palatalis. Tuba gradually narrowed towards the constriction, angular or with an almost sharp ridge below. Radial ribs on the spire (rather) closely placed (6-10 ribs/0.5 mm on the penultimate whorl), not sinuous, or very slightly so on the lower half of the penultimate whorl; but those on the last whorl closely placed to widely spaced (2-12 ribs/0.5 mm half-way), with a single deep loop on the periphery when the shell is observed in front view; those on the tuba closely placed to widely spaced (2-12 ribs/0.5 mm half-way), below with a short, shallowly concave to deeply trough-shaped projection abrading to a slightly sinuous to deeply looped scar. Spiral striation absent or inconspicuous. Umbilicus open, 0.15-0.20 mm across. Aperture tilted up to 30° with regard to the coiling axis, its upper margin below the level of the apex, usually elliptic. Peristome (almost) touching the spire, sometimes distant from it, double; outer peristome widely spreading beyond the inner, but narrowed towards, and absent along the right side of the aperture, along the upper side of the aperture widened or with a an obtuse (rounded but not widely so) wing which is sometimes truncated on its right side, along the left and the lower side often with a similar but less distinct wing ; inner peristome slightly to moderately protruding from the outer, spreading. Spire; height 1.8-2.5 mm; width 1.3-1.5 mm; index 1.4-1.7. Total width 2.3-3.1 mm. Height aperture 0.6-0.7 mm; width 0.7-0.8 mm.
Distribution. — Borneo: Sarawak, 1st Div., limestone ranges S. of Kuching, on widely scattered localities.

Specimens with the aperture not touching the spire, and with a distinct wing along the upper side of the aperture (e.g. UF 194841 from the Serian area) differ from 0. heteropleuron in having a spire with sinuous radial ribs.