Plectostoma simplex (Fulton, 1901)
“This form is nearest in its general characters to O. austeni Smith, but can be readily distinguished from that species by its smaller size and different peristome, which in austeni is rather thick and has practically no flange; whereas in simplex the peristome is thin and the flange produced, especially at its upper part. The aperture of O. austeni is somewhat angular, not circular as in O. simplex.” (Fulton, 1901)
“The minute size (height 1.8 mm), weak ribs and appression of the trumpet to the preceeding whorl identify this species.” (Solem, 1964)
Fulton (1901) original descriptions on Opisthostoma simplex – “Shell pyramidal, narrowly perforate, apical whorls reddish, lower whorls of a somewhat shining golden colour; whorls 6, convex, slowly increasing, ornamented with close oblique costae; aperture circular; peristome thin, slightly expanded, not free, right-hand margin touching the whorls, bordered at the inner margin with a narrow thin rim or flange which is expanded at the upper part.
Kobelt (1902) descriptions on Opisthostoma (Plectostoma) simplex – “Schale eng durchbohrt, kegelförmig, etwas goldgiänzend, mit rötlicher Spitze; 6 gewölbte, langsam zunehmende, dicht schräg gerippte Windungen; Mündung kreisrund; Mundrand dünn, leicht ausgebreitet, nicht lostretend, rechts angelötet, links mit einem nach oben verbreiterten Randsaum.”
By Vermeulen (1994)
"Spire conical with slightly concave to slightly convex sides. Apex not oblique. Whorls 5 1/8-6 3/8, convex; last whorl rounded to slightly angular. Constriction with a parietalis, a longitudinal and a transverse palatalis, a columellaris which continues as a knob- to ridge-shaped transverse basalis. Tuba gradually narrowed towards the constriction, rounded below. Radial ribs on the spire rather closely placed to moderately spaced (4-5 ribs/0.5 mm on the penultimate whorl), sinuous or not, but those close to the tuba not sinuous; those on the tuba moderately spaced (3 ribs/0.5 mm half-way), slightly sinuous below. Spiral striation present, often inconspicuous. Umbilicus open, 0.25-0.40 mm across. Aperture tilted up to 30º with regard to the coiling axis, cricular, elliptic or slightly angular. Peristome touching the spire or distant from it, double; outer peristome spreading beyond the inner, gradually to rather abruptly narrowed towards, and absent along the right side of the aperture, usually with a distinct, obtuse (rounded but widely so) wing along the upper side, and sometimes a similar but less conspicuous wing along the lower side; inner peristome moderately protruding from the outer, often with a few protruding lamellae on its outer surface, spreading. Spire: height 1.5-2.2 mm; width 1.2-1.6 mm; index 1.2-1.5. Total width 2.0-2.5 mm. Height and width aperture 0.5-0.6 mm."
Opisthostoma (Plectostoma) simplex – “H. 1.75, Durchm. 2 mm.” (Kobelt, 1902)
Type locality – “Gomanton, N. Borneo” (Fulton, 1901)
Kinabatangan valley, Sabah, Malaysia.