“This new species is so little irregular that one might take it for a member of a different genus, e.g. Palaina. The general architecture and inconspicuous deviation in the last whorl, however, place it in Opisthostoma. From the related specis O. (P.) senex, O. (P.) sciaphilum and O. (P.) turriforme it can be distinguished by the wide umbilicus.” (Benthem-Jutting, 1952)
Benthem-Jutting (1952) original descriptions on Opisthostoma (Plectostoma) umbilicatum – “Shell turreted, yellowish or reddish-brown. First 1 ½ whorl smooth, subsequent ones ornamented with fine oblique ribs, distantly placed and standing out wing-like away from the shell. Between the ribs there is a fine spiral striation. Not transparent or shining. Whorls 6 ½-7 ½, convex. Top whorls only slightly oblique on the shell axis. The fifth whorl is the broadest of all, the sixth does not reach beyond the fifth and is usually less wide. Abaout midway in the last whorl there is an inconspicuous tranverse constriction. After this the whorl widens again towards the aperture and is slightly irregular in its course, although no so bizarre as in O. (P.) salpidomon and preceding species. Adnate to the penultimate whorl. Suture deep. Umbilicus open, wide. Aperture round, vertical. Peristome continuous, almost circular, with a double margin. Operculum unknown.”
Opisthostoma (Plectostoma) umbilicatum – “Dimensions: Height 2.8-3.1, Breadth 1.4-1.6, Diam. apert. 0.8-1.0 mm.” (Benthem-Jutting, 1952)
Type locality – “Malaya: Kota Tongkat, Pahang” (Benthem-Jutting, 1952)