Plectostoma picsingense (Smith, 1905)
“In this species the last whorl ascends so that the top of the peristome is on a level with the top of the third whorl. There does not appear to be any spiral sculpture.” (E. A. Smith, 1905)
E. A. Smith (1905) original descriptions on Opisthostoma picsingense – “Testa conica, rimata, rufescens, dextrorsa; anfractus 6 convexi, regulariter accrescentes, lamellis numerosis, tetiuissimis, confertis, obliquis, ornati, ultimus ad peripheriam obtuse subangulatus. antice contortus et retrorsus ascendens, infra cristatus; apertura rotundata, campanulata; peristoma duplex, pallidum, margine interno continuo, leviter expanse, externo versus spiram fere interrupto, anfract. penultimo contiguo.”
By Vermeulen (1994)
"spire conical with slightly to distinctly concave sides. Apex not oblique. Whorls 5 1/4 - 5 7/8; top whorls convex; next whorls slightly conxev; last whorl almost sharply angular at the periphery, almost flat to slightly concave above, almost flat below. Constriction with a parietalis, a transverse palatalis. Tuba gradually narrowed towards the constriction, angular or with an almost sharp ridge below. Radial ribs on the spire (rather) closely placed (6-10 ribs/0.5 mm on the penultimate whorl), not sinuous, or very slightly so on the lower half of the penultimate whorl; but these on the last whorl closely placed to widely spaced (2-12 ribs/0.5 mm half-way), with a shingle deep loop on the periphery when the shell is observed in front view; those on the tuba closely placed to widely spaced (2-12 ribs/0.5 mm half-way), below woth a short, shallowly concave to deeply trough-shaped projection abrading to a slightly sinuous to deeply looped scar. Spiral striation absent or inconspicuous. Umbilicus open, 0.15-0.20 mm across. Aperture tilted up to 30 degrees with regard to the coiling axis, its margin below the level of the apex, usually elliptic. Peristome (almost) touching the spire, sometimes distant from it, double; outer peristome widely spreading beyond the inner, but narrowed towards, and absent along the right side of the aperture, along the upper side of the aperture widened or with a an obtuse (rounded but not widely so) wing which is somethings truncated on its right side, along the left and the lower side often with a similar but less distinct wing; inner peristome slightly to moderately protruding from the outer, spreading. Spire: height 1.8-2.5l width 1.3-1.5 mm; index 1.4-1.7. Total width 2.3-3.1 mm. Height aperture 0.6-0.7 mm; width 0.7-0.8 mm."
Opisthostoma picsingense – “Longit. 2.3, diam. 2.6 mm.” (E. A. Smith, 1905)
Type locality – “Picsing, Upper Sadong, Sarawak” leg. S. Beddome (E. A. Smith, 1905)