Opisthostoma fallax van Benthem Jutting, 1961
“From O. (O.) castor and O. (O.) pollux the new species differs in the more oblique shape, the wider umbilicus and the more twisted terminal part of the last whorl. O. (O.) fallax bears some resemblance to O. (O.) michaelis v. B. Jutting, but this species is smaller, less oblique and has a narrower umbilicus.” (Benthem-Jutting, 1961)
Benthem-Jutting (1961) original descriptions on Opisthostoma (Opisthostoma) fallax – “Shell white, obliquely cylindrical, more oblique than in O. (O.) castor v. B. Jutting and O. (O.) pollux v. B. Jutting. First whorl smooth, following ones ribbed with fine, white, tranverse ribs, close-set in the earlier whorls, but wider apart in the last one. No spiral striation visible. Whorls 4, convex. The top whorls almost in one plane with the third whorl, and placed obliquely on the axis of the shell. Two last whorls convex, the third whorl, and placed obliquely on the axis of the shell. Two last whorls convex, the third distinctly bulging out in the shell profile, although not so much as in O. (O.) micridium n.sp. Suture deep. Terminal part of last whorl first constricted, then widening again, continuing in a double bend towards the aperture. Aperture rather small, directed obliquely towards the sky, adnate to penultimate whorl, but not reaching higher than the middle of the last whorl. Umbilicus open, wider than in O. (O.) castor, O. (O.) pollux or O. (O.) micridium. Peristome continuous, circular, sometimes duplex. Operculum unknown.”
Opisthostoma (Opisthostoma) fallax – “Dimensions: Height 0.65-0.8 mm, Width 1.2-1.35 mm, Diam. apert. 0.35-0.4 mm.” (Benthem-Jutting, 1961)
Type locality – “Malaya: Sungei Siput, Perak” (Benthem-Jutting, 1961)