Material seen. — SARAWAK. 1st Div.: SE. end of limestone outcrop S. of Bau (leg. Wilford, UF 194694/>10, inch HOLOTYPE; do. I94720/>10; do. 194936/1).
Spire cylindrical. Apex distinctly oblique. Whorls 3 1/8-3 3/8, convex; last whorl rounded. Constriction with a transverse palatalis. Tuba touching the spire over its entire length, close to the constriction with a distinct swelling without radial ribs, bordered by a depression on both sides; the swelling with a deep semi-circular furrow opening to the right side if the shell is observed in front view; tuba rounded below. Radial ribs not sinuous, on the spire moderately spaced (4-6 ribs/0.5 mm on the penultimate whorl), those on the tuba often slightly wider spaced (4 ribs/0.5 mm half-way the tuba). Spiral striation present. Umbilicus usually entirely covered by the tuba. Aperture hardly tilted with regard to the coiling axis, its upper margin usually slightly above the level of the apex, circular to slightly elliptic. Peristome double, outer peristome spreading beyond the inner, but rather abruptly narrowed towards, and absent along the (lower) right side of the aperture, widened along the upper (right) side, and, slightly less so, along the lower side; inner peristome distinctly protruding from the outer, somewhat spreading. Spire: height 0.9-1.0 mm, width 0.7-0.9 mm; index 1.1-1.3. Total width 1.1-1.3 mm. Height aperture 0.3-0.4 mm; width 0.35-0.40 mm.
Distribution. — Borneo: Sarawak, 1st Div., limestone range SE. of Bau.
Notes. — 1. Characterized by the swelling in the tuba, with a semi-circular furrow on top of it. The name refers to this furrow. 0. shelfordi has a similar swelling, including the furrow. It differs in having a conical spire.