“O. (P.) senex although closely related to O. (P.) sciaphilum and O. (P.) turriforme is different in having a plumper shell, of a livid greyish hue and with the ribs standing about twice as wide.” (Benthem-Jutting, 1952)
Benthem-Jutting (1952) original descriptions on Opisthostoma (Plectostoma) senex – “Shell turreted, brownish-grey to greyish, not shining or transparent. Initial 2 whorls smooth, following ones ornamented with oblique fine ribs, white or of the same colour as the shell. Especially on the penultimate and ultimate whorls, before the constriction, there ribs are distinctly placed, and they stand out wing-like away the the shell. There is no spiral sculpture between the ribs. Whorls 7-8, well rounded, regularly increasing in size. Suture deep, Top whorls not obliquely implanted on the rest of the shell. Last whorl about midway somewhat constricted transversely, then widening again towards the aperture and somewhat irregularly coiled, although not so bizarre as in O. (P.) salpidomon and preceding species. Adnate to previous whorl. In some specimens the aperture and part of the last whorl are orientated as in a normal dextral shell. In others, however, the aperture deviates so as to give a false impression of a sinistral shell. Umbulicus as a narrow slit, or closed by the last whorl hiding the opening. Aperture round, vertical or facing obliquely downwards. Peristome continuous, circular, appearing double by the presence of a wing-like collar behind the peristome and parallel to it. Operculum unknown.”
“In O. michaelis v. B. Jutting, O. kakiense Tomlin and O. senex v. B. Jutting the spiral sculpture is definitely absent.” (Benthem-Jutting, 1961)
Opisthostoma (Plectostoma) senex – “Dimensions: Height 3.4-4.0, Breadth 1.6-1.8, Diam. apert. 0.9-1.2 mm.” (Benthem-Jutting, 1952)
Type locality – “Malaya: Bukit Charas, near Kuantan, Pahang” (Benthem-Jutting, 1952)