Material seen. — KALIMANTAN. Kalimantan Selatan: G. Siamang near Desah Liu, 30 km E. of Tandjung (V 3096/1);
Spire conical with flat to concave sides, rarely with very slightly convex sides. Apex slightly oblique or not. Whorls 4 5/8-6 7/8, convex; last whorl rounded or slightly angular. Constriction with a sharply outlined, regularly shaped parietalis, a similar longitudinal palatalis, a transverse palatalis, a columellaris gradually thinning out towards the basal side of the constriction. Tuba gradually narrowed towards the constriction, rounded below. Radial ribs (moderately) spaced (3-5 ribs/0.5 mm on the penultimate whorl, 3-4 ribs/0.5 mm half-way on the tuba), (slightly) sinuous (rarely not so); those on the tuba not sinuous. Spiral striation fine. Umbilicus open, 0.20-0.35 mm across. Aperture tilted up to 30° with regard to the coiling axis, circular to subrectangular. Peristome distant from the spire, double; outer peristome hardly to moderately spreading beyond the inner, but gradually narrowed towards, and absent narrow along the right side of the aperture, often slightly widened along the upper side; inner peristome slightly protruding (rarely somewhat more protruding) from the outer, spreading. Spire: height 1.5-2.3 mm; width 0.9-1.5 mm; index 1.3-1.7. Total width 1.7-3.0 mm. Height and width aperture 0.4-0.6 mm.
Distribution. — Borneo: Kalimantan, SE. and E. part.

1. O. decrespignyi usually has straight radial ribs, as well as a columellaris which abruptly ends towards the basal side of the constriction.
2. The name refers to the fact that this is the only species of subg. Plectostoma occurring S. of the equator.