Apex. Shape: distinctly convex.
Spire. Height: 1.9–2.1 mm. Width: 1.2–1.5 mm. Number of whorls: 4 3/4–5. Apical spire shape: oblong conical. Basal spire shape: conical. Whorl periphery: distinctly convex. Umbilicus: Open.
Constriction. Parietal teeth: two. Basal teeth: none.
Tuba. Coiling direction: type 1 and aperture visible between right lateral and back view. Tuba whorl length in proportion to spire last whorl: ca. 3/8–1/2. Proportion of tuba that attaches to spire: at least 1/2.
Aperture and peristome. Peristome: double peristomes. Outer peristome shape: same as inner peristome and uniformly projected all around, except the posterior part.
Spiral lines. Thick lines: present. Thin lines: present.
Radial ribs. Rib density: 7–8 ribs per mm. Rib intensity: thin. Shape: slightly curved. Inclination: orthoclin.
to be added
Holotype: ZMA 136054(1) (Seen). Paratype: ZMA 136055(8) (Seen).
See discussion under Plectostoma christae.
Type locality. Gua Siput, Taman Negara, Pahang (4°26'47"N, 102°14'44"E).
Distribution range. Plectostoma siphonostomum has a similar distribution pattern asPlectostoma salpidomon and often occurs sympatrically with that species. It can be found in many limestone outcrops in the valley between the Titingwangsa Range, Tahan Range and Benom Range (Figure 18D).