“The upper whorls of this species are sometimes pale and sometimes reddish; this, however, may be due to staining from the animal. The last whorl is invariably reddish, especially the first half. It is closely related to O. otostoma, but may be distinguished by its less convex whorls, the different form of the aperture, and the more thickened peristome, which is scarcely lobed above, and also by the body-whorl being adnate to the spire, whereas in O. otostoma it is quite free.” (E. A. Smith, 1894)
“Characterised by the peculiar shape of the aperture, which is distinctly channelled at the lower part.” (E. A. Smith, 1905)
E. A. Smith (1894) original descriptions on Opisthostoma austeni – “Testa dextrorsa, conica, angustissime perforata, pallida vel plus minus rufescens; anfractus 6, convexi, regulariter crescentes, sutura profunda sejuncti, liris obliquis tenuissimis curvatis (in anfr. ultimo sensim crassioribus et distantioribus) instructi; anfr. ultimus constrictus, ad aperturam valde ascendens et retroversus, haud solutus; apertura rotunde angulata; peristoma leviter incrassatum, porrectum, vix expansum, extus lamella tenui angusta circumdatum.”
E. A. Smith (1905) descriptions on Opisthostoma sadongense – “Testa dextrorsa, rimata, conica, rufescens; anfractus 6 perconvexi, lamellis numerosis, tenuibus, obliquis, ornati, ultimus constrictus, retrorsus contortus et ascendens, lamellis versus labrum validioribus et magis distantibus; apertura angulatim ovalis, antice leviter canaliculata; peristoma duplex, margine interno continuo, porrecto, leviter incrassato, externo anguste expanso, spiram versus interrupto.”
[Jaap Vermeulen, 1994]
Spire conical with flat to slightly convex sides. Apex (slightly) oblique. Whorls 4 5/8 - 5 5/8, convex; last whorl rounded. Constriction rarely with an inconspicuous parietalis, with a transverse palatalis. Tuba touching the spire over its entire length, gradually narrowed towards the constriction, rounded below. Radial ribs on the spire rather closely placed to widely spaced (3-8 ribs/0.5 mm on the penultimate whorl), not or slightly sinuous, but those on the last whorl no sinuous; those on the tuba (widely) spaced (2-4 ribs/0.5 mm half-way), not sinuous below, often with much finer radial riblets in between. Spiral striation absent. Umbilicus open, 0.15-0.25 mm across. Aperture tilted up to 30 degree with regard to the soiling axis, its upper margin sidely below the level of the apex, subtriangular with an obtusely angular to deeply grooved lower edge, an obtusely angular upper edge, an obtusely angular to slightly rounded right edge, the left side well rounded. Peristome double; outer peristome slightly to moderately spreading beyond the inner, but gradually narrowed towards, and sometimes along the lower; inner peristome (moderately) protruding from the outer, somewhat spreading. SPire: height 1.8-2.2 mm; width 1.3-1.5 mm; index 1.3-1.6. Total width 2.3-2.7 mm. Height aperture 0.8-0.9 mm; width 0.5-0.7 mm.
Opisthostoma sadongense – “Longit. 2.25, diam. 2.5 mm.” (E. A. Smith, 1905)
Shell height – 2.5 mm; Shell width – 1.5-2.75 mm. Aperture diameter including peristome – 1.25 mm.
Type locality – “Rumbang, Sarawak” (E. A. Smith, 1894) (1.268388,110.263839)
“Picsing, Upper Sadong, Sarawak” leg. S. Beddome (E. A. Smith, 1905)
Kpg. Benuk, Padawan, Sarawak, Malaysia; Fairy Caves, 8km from Bau, Sarawak, Malaysia; Mount Jambusan, 4km from Bau, Sarawak, Malaysia