Material seen. — SABAH. Sandakan Zone: limestone escarpment along Sg. Tabin, lower Segama Valley (leg. Wilford, UF 194801 />10). Tawau Zone: G. Madai, 40 km SSW. of Lahad Datu (leg. Dorman, UF 196754/>10; V 1702/>I0, HOLOTYPE RMNH 56920).

As the type subspecies, but spire with slightly concave to convex sides. Constriction with a distinctly knob-shaped basalis. Radial ribs on the spire rather closely placed to moderately spaced (3-6 ribs/0.5 mm on the penultimate whorl), slightly sinuous, but those close to the tuba often not or hardly sinuous; those on the tuba moderately to widely spaced (2-4 ribs/0.5 mm half-way). Umbilicus 0.30-0.40 mm across. Aperture tilted up to 80° with regard to the coiling axis. Peristome (widely) distant from the spire, outer peristome somewhat spreading beyond the inner or not, sometimes slightly widened along the upper side of the aperture; inner peristome hardly protruding from the outer. Spire: height 1.6-2.1 mm; width 1.2-1.4 mm; index 1.2-1.5. Total width 2.0-2.6 mm. Height and width aperture 0.5-0.7 mm.
Distribution. — Borneo. Sabah, SE. part, on two widely distant limestone hills.

1. Differs from lissopleuron by usually having a distinctly knob-shaped basalis. Some specimens of lissopleuron have a similar but less conspicuous basalis, but these usually have the outer peristome distinctly spreading beyond the inner. Specimens of subspec. bigibbum often have a longer tuba than the type subspecies.
2. 0. tuba does not have a longitudinal palatalis in the constriction; 0. decrespignyi dots not have a basalis.
3. The name refers to the two knobs present along on the basal side of the constriction.