“The curious free trumpet of the last whorl, not ascending as in other members of the genus Opisthostoma, but descending spirally makes this new species easily distinguished from any of its congeners.” (Benthem-Jutting, 1952)
Benthem-Jutting (1952) original descriptions on Opisthostoma (Plectostoma) siphonostomum – “Shell turreted, straw-colour, fawn or light pinkish-brown. First whorl smooth, subsequent ones ornamented with fine white vertical ribs standing out wing-like away from the shell. On the spire the ribs are rather distantly placed, on the free part of the last whorl, however, closer together. In fresh shells each rib is most developed in the middle of each whorl, thus rendering the profile falsely angular. No spiral sculpture is visible. Shell a little shining and transparent. Whorls 6-6 ½, convex. The first 5 ½-6 whorls form a normal high spire, the second part of the last whorl, however, deviates from the regular course. It is free from the preceding whorl and descends more rapidly, as in certain species of the genera Tortulosa, Diaphera or Brachypodella. At the starting point of the irregular part of the last whorl the shell is distinctly contricted transervsely. Beyond that point it widens again to the aperture. Suture deep. Umbilicus open, moderately wide. Aperture round, oblique. Peristome continuous, circular, slightly thickened and little reflected, with a double margin. Operculum unknown.”
“…on renewed investigation, spiral striae proved to be present.” (Benthem-Jutting, 1961)
The shell is small, brownish to pinkish white, shining and somewhat transparent with turreted spire and circular in cross-section. Whorls 6 to 6 ½ (tuba is not included), convex with deep suture. The spiral striation is absent. The first whorl are smooth, and following whorls ornamented with thin, vertical, and wing-like ribs which and widely spaced (four ribs/0.5 mm on the penultimate whorl and five ribs/0.5 mm on the body whorl close to the constriction). Umbilicus is moderately wide and approximately 0.2 mm across. The body whorl is constricted with transverse palatalis. The tuba is completely detached after the six whorls, and projecting downwards. Aperture is round and oblique with double rounded peristome; the inner peristome is slightly reflected and protruding distinctly from the outer peristome. Operculum is unknown. The shell height is between 2.9-2.5 mm, the total width is between 1.5-1.4 mm, and aperture diameter is between 0.8-0.7 mm.
Opisthostoma (Plectostoma) siphonostomum – “Dimensions: Height 2.5-2.9, Breadth 1.4-1.5, Diam. apert. 0.7-0.8 mm.” (Benthem-Jutting, 1952)
Type locality – “Malaya: Goa Siput, near Batu Lompat, Pahang” leg. C. S. Ogilvie (Benthem-Jutting, 1952)
Other localities – “Malaya: Gua Che Yatin, Ulu Tembeling, Pahang” (Benthem-Jutting, 1961)
PAHANG - Taman Negara; Gua Che Yatin, Ulu Tembeling (Benthem-Jutting, 1962); Taman Negara; Gua Siput, near Batu Lompat (Leg. C.S. Ogilvie/1949); Chegar Perah, along Kuala Lipis-Gua Musang road (Leg. T.S. Liew & M.E. Marzuki/2010)