Ancey (1887) original descriptions on Plectostoma wallacei – “Testa elevato-conica, sat tenuis, albida; spira apice subacuta; anfractus 5 lente regulariterque crescentes, sutura profunda fere horizontali discreti; primi 2 nitidi, lacvigati; tertius obsoletissime confertimque costulato-substriatus; sequentes costulis prope suturam valde obliquis, deinde abrupte angulatimque verticaliter inflexis et flexuosis eleganter lamellosi; his costulis minulis, valde approximatis et numerosis; ultimus anfractus more typicae speciei (P. decrespignyi H. Ad.), distortus; parte soluta angulo basali cristato-crenulata, confertissime verticaliter lamellosa. - Umbilicus apertus. - Apertura rotundata, vix obliquatula. - Peristoma continuum, undique subincrassatum, expansum.” E. A. Smith (1905) descriptions on syntype Opisthostoma beddomei – “Testa minima, angustissime perforata, rufescens, dextrorsa, pyramidalis; anfractus 6 convexi, lamellis tenuissimis, subremotis, obliquis, in medio undulatis, ornati, supremi 1-2 laeves, saturate rufescentes, ultimus infra carino-cristatus, lamellis confertis instructus, antice solutus, retrorsus, et sursum contortus; apertura subcircularis, intus pallide rufescens; peristoma duplex, albidum, margine interno tubaeformi, expanso, externo aliquanto angusto, dextrorsum versus spiram interrupto.” Kobelt (1902) descriptions on Opisthostoma (Opisthostoma) wallacei – “Schale offen genabelt, hoch kegelförmig, ziemlich dünn, weisslich; Gewinde ziemlich spitz; Naht tief, fast wagerecht; 5 langsam und regelmässig zunehmende Windungen, die 2 obersten glänzend, glatt, die dritte undeutlich rippenstreifig, die folgenden mit oben schrägen, dann plötzlich im Winkel senkrecht herabgebogenen, bogigen, lamellösen Rippen, letzte Windung verdreht, vorn abgelöst, mit einem gezähnelten Basalkamm und sehr dichten Radiallamellen; Mündung abgerundet, kaum schräg; Mundrand zusammenliängend, ringsum leicht verdickt und ausgebreitet.”
By Vermeulen (1994)
" Spire conical with flat to convex (sometime with slightly concave) sides. Apex slightly oblique or not. Whorls 4 1/4- 5 5/8, convex; last whorl rounded, or sometimes slightly angular at the periphery, slightly convex above and below. Constriction with an inconspicuous to distinct parietalis, with or without a longitudinal palatalis, with a transverse palatalis. Tuba free from the spire or touching it over its entire length, gradually narrowed towards the constriction, rounded below. Radial ribs on the spire rather closely placed to swidely spaced (3-7 ribs/0.5 mm on the penultimate whorl), (slightly) sinuous about half-way, but those close to the tuba (slightly) sinuous or with a single or a double, deep or shallow loop when the shell is observed in front view; those on the tuba closely placed to widely spaced (3-9 ribs /0.5 mm half-way), sinuous below or not, or with a shallow to deep, semi cricular loop, in the latter case often with a short, shallowly concave to deeply trough-shaped projection. Spiral striation present, often inconspicuous, very fine, rarely entirely absent. Umbilicus open, 0.10-0.20 mm across. Aperture tilted up to 30 degree with regard to the coiling axis, its upper margin widely below the level of the apex, slightly angular to subtriangular with a narrowly rounded lower edge and a slightly wider rounded upper and right edge, the left side well rounded. Peristome touching the spire or distant from it, double, outer peristome moderately to sidtinctly spreading beyond the inner, but gradually narrowed towards, and absent along the right side of the aperture, sometimes slightly widened along the upper and the lower side; inner peristome moderately to distinctly protruding from the upper surface of the outer peristome usually without, but sometimes with some fine riblets or lamellae. Spire: height 1.1-2.0 mm; width 0.8-1.2 mm; index 1.3-1.6. total width 1.3-2.4 mm. Height and width aperture 0.4-0.7 mm"
Plectostoma wallacei – “Long. 2; lat. (cum parte in lubam extùs porrecta) 2½; diam. min., 1 mill. ½.” (Ancey, 1887); Opisthostoma (Opisthostoma) wallacei – “H. 2. Durchm. 2.5 : 1.5 mm.” (Kobelt, 1902); Syntype Opisthostoma beddomei – “Long. 1.75, diam. max. 2.25 mm.; apertura cum perist. 1 m. lata.” (E. A. Smith, 1905)
Type locality – “Borneo” leg. G. B. Sowerby (Ancey, 1887); syntype Opisthostoma beddomei “Bidi Mountains, 20 miles from Sarawak” (E. A. Smith, 1905) Other localities – “Busau, N. W. Borneo” (E. A. Smith, 1894)
Mount Jambusan, Bau area, Sarawak, Malaysia; Fairy Caves, 8km from Bau town. Wind Caves, 6km from Bau town. Widely distributed in Bau areas. Also recorded from Lubang Angin, Serian-Tebakang road, Sarawak, Malaysia.