“The species is very similar to O. umbilicatum van Benthem-Jutting, 1952, but its shells have one whorl less and are smaller (O. umbilicatum, H 2.8-3.1 mm) and slender ovoid instead of turreted.” (Maassen, 2001)
Maassen (2001) original descriptions on Opisthostoma (Plectostoma) sinyumensis – “Shell dextral, ovoid, light brown, shining and transparent; with 5.5 distinctly convex whorls, separated by a rather deep suture, and provided with a distinct, very fine, spiral striation. Last whorl about as broad as the penultimate one. Radial ribs rather thin, increasingly higher and sharper on the lower whorls; on the penultimate whorl 7-8 ribs/mm, above the aperture 8-9 ribs/mm. The constriction consists only of transversal thickening on the palatal side of the body-whorl. The tuba is 0.75 whorl long and turns off slightly inwars. Umbilicus very narrow. Peristome double; the inner peristome protruding distinctly from the outer one. In frontal view, the aperture touches the middle of the previous whorl and is hardly turned downwards. Operculum unknown.”
Opisthostoma (Plectostoma) sinyumensis – “H 1.9-2.2 mm; B 1.2-1.3 mm.” (Maassen, 2001)