Material seen. — KALIMANTAN. Kalimantan Selatan: Nateh near Batu Tangga, approximately 18 km E. of Barabai (V 3016/>10, HOLOTYPE RMNH 56913).
Spire cylindrical. Apex (slightly) oblique. Whorls 3 1/4-3 1/2, convex; last whorl rounded. Constriction with a transverse palatalis. Tuba about half-way leaving a small gap between the tuba and the spire, gradually narrowed towards the constriction, rounded below. Radial ribs moderately spaced (4-6 ribs/0.5 mm on the penultimate whorl as well as half-way the tuba), not sinuous. Spiral striation present, fine. Umbilicus open, 0.1 mm across. Aperture tilted up to 60° with regard to the coiling axis, its upper margin usually widely above the level of the apex, somewhat elliptic. Peristome touch¬ing the spire, double, outer peristome spreading beyond the inner, but gradually narrowed towards, and absent along the right side of the aperture; inner peristome distinctly protruding from the outer, somewhat spreading. Spire: height 0.8-0.9 mm;
width 0.7-0.8 mm; index 1.1-1.3. Total width 1.1-1.2 mm. Height aperture 0.25-0.35 mm; width 0.35-0.40 mm.
Distribution. — Borneo: Kalimantan, K. Selatan, W. flanks of the Meratus Mts.
Notes. — 1. Similar in some aspects to 0. acolaston; see the note under that species.
2. The name refers to the partially free tuba.