“The new species differs from O. (O.) castor and O. (O.) pollux in the slightly smaller size and the more oblique shape. It has the greatest affinities to O. (O.) pollux, but the aperture of O. (O.) micridium does not reach so high up against the penultimate whorl, and the trumpet-shaped part of the last whorl is less twisted.” (Benthem-Jutting, 1961)
Benthem-Jutting (1961) original descriptions on Opisthostoma (Opisthostoma) micridium – “Shell white, obliquely low-cylindrical, more oblique than in O. (O.) castor v. B. Jutting and O. (O.) pollux v. B. Jutting. First whorl smooth, following ones finely ribbed with white, tranverse ribs all over the shell. No spiral striation. Whorls 3¾, convex. The top whorls almost in one plane, slightly oblique on the axis of the shell. The two last whorls convex, the third one bulging out far in the shell profile. Last whorls a little flattened and broadened at the basal side. Suture deep. Last whorl a little flattened and broadened at the basal side. Suture deep. Last whorl directed upwards and adnate, with almost no deviation. It reaches till about midway up the height of the preceding whorl. Umbilicus open. Aperture rounded, obliquely directed to the sky. Peristome continuous, circular to rounded-triangular, duplex. Operculum unknown.”
Opisthostoma (Opisthostoma) micridium – “Dimensions: Height 0.6-0.7 mm, Width 1.0-1.2 mm, Diam. apert. 0.3-0.4 mm.” (Benthem-Jutting, 1961)
Type locality – “Malaya: Gunong Sinyum, Pahang” (Benthem-Jutting, 1961)