Plectostoma otostoma (Boettger, 1893)
By Boettger (1893)
"Differt. Ab O. crespignyi H. Ad. T. majore, apert. Inverse auriformi, perist. Superne angulatim valde protracto. - T. sinistrorsa, anguste umbilicata, elongato-conica, tenera, pallide fulva, sursum sanguinea; spira conica, lateribus fere strictis; apex obtusulus. Anfr. 6 1/2 perconvexi sed haud angulati, sutura profunda sejunti, eleganter tenuilamellati, lamellis compresses, distantibus, albidis, ultimus initio constrictus, tum tubam solutam, sigmoideam, sinistrorsum simulque sursum flexum efficians. Apert. libera subverticalis, inverse auriformis, intus circularis; perist. duplicatum, externum expansum, inferne subangulatum, superne in lobum rotundata-triangularem latere dextro emarginatum elongatum. Alt. 2 1/4, diam. max 2 1/2 mm; alt/ apert. 1 1/4, lat. apert. 1 mm."
By Vermeulen (1994)
"Spire conical with flat sides. Apex slightly oblique. Whorls 5 1/2 - 6 1/8, convex; last whorl rounded. Constriction with a longitudinal and a transverse palatalis, and an inconspicuous columellaris. Tuba somewhat abrupt narrowed towards the constriction, rounded. Radial ribs moderately spaced (3-4 ribs/0.5 mm on the penultimate whorl, 3 ribs/0.5 mm half-way the tuba), on the spire slightly sinuous, those close to the tuba not sinuous; those on the tuba not or slightly sinuous below. Spiral striation present. Umbilicus open, 0.15-0.30 mm across. Aperture hardly tilted with regard to the coiling axis, circular to elliptic. Peristome double; outer peristome spreading beyond the inner but gradually narrowed towards, and absent along the right side of the aperture, with a distinct, but small wing along the upper side which often has a somewhat concave right margin; inner peristome slightly protruding from the outer, spreading. Spire: height 2.0-2.2 mm; width 1.5-1.6 mm; index 1.3-1.4. Total width 2.4-2.7 mm. Height aperture 0.6-0.7 mm; width 0.6 mm."
Brunei.
Note: The exact location is likely at Mulu, limestone hills. Based on the description by Boettger (1893) "On a high limestone mountains in Brunei, N. W. Borneo, in small numbers, with O. pulchellum Godw. - Aust., O. baritense E. A. Smith, Diplommatina everetti E. A. Smith, D. baritensis EA Smith and other small snails come together and 0 of Dr. Staudinger & A. Bang-Haas in Blasewitz Dresden me to determine posted."