“Well characterized by the distinctly elongated, curved inner peristome. Except for its inner peristom, the sympatrical O. semisolutum differs in the absence of a furrow and a swelling on the proximal part of the tuba, and having a wider umbilicus, as well as in the outer peristome not covering the apex.” (Vermeulen, 1994)
This species has longest tuba proportioned to shell height where the tuba extent over the shell apex and bending downward.
Vermuelen (1994) original descriptions on Opisthostoma acolaston – “Spire cylindrical. Apex slightly oblique. Whorls 3½-37/8, convex; last whorl rounded. Constriction with a tranverse palatalis. Tuba about half-way leaving a small gap between the tuba and the spire, with a slight, vertical depression at about 0.2 mm from the constriction and a swelling in between, 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 slightly wider spaced (4 ribs/0.5 mm half-way the tuba). Spiral striation, or very fine. Umbilicus up to 0.1 mm across, but often entirely covered by the tuba, Aperture tiltcd up 90° with regard to thc coiling axis (facing downwards), its upper margin usually widely below the level of the apex, circular to slightly angular. Peristome double, outer peristome touching the spire, moderately spreading beyond the inner, but gradually widened towards the right side of the aperture and often partly covering the apex. inner peristome widely projecting beyond the outer, forming a downwards curvedtube of up to 2/3 of the length of the tuba, somewhat spreading, its outer surface with rather closely placed to moderately spaced radial ribs.”
Opisthostoma acolaston – “Spire: height 0.8-1.0 mm; width 0.8-0.9 mm: index 1.0-1.3. Total width about 1 mm. Height and width aperture 0.35-0.40 mm.” (Vermeulen, 1994)
Type locality – “Kalimatan Selatan, Nateh near Batu Tangga, approximately 18 km E. of Barabai” leg. Vermeulen (Vermeulen, 1994)
So far the this species has been recorded from Southern part of Kalimantan, Indonesia. Island of Borneo.