Apex. Shape: slightly to moderately convex.
Spire. Height: 1.6–2.0 mm. Width: 1.3–1.5 mm. Number of whorls: 3 5/8–4 1/4. Apical spire shape: depressed conical. Basal spire shape: ovoid. Whorl periphery: moderately to distinctly convex. Umbilicus: open or partially closed by tuba.
Constriction. Parietal teeth: none. Basal teeth: none.
Tuba. Coiling direction: type 3 and aperture visible in right lateral view. Tuba whorl length in proportion to spire last whorl: ca. 5/8–3/4. Proportion of tuba that attaches to spire: whole.
Aperture and peristome. Peristome: double peristomes. Outer peristome shape: similar to inner peristome, projected all around, except the posterior part, where the left lateral sides are slightly more projected than the anterior and right lateral side.
Spiral lines. Thick lines: absent. Thin lines: present.
Radial ribs. Rib density: 10–11 ribs per mm. Rib intensity: thin. Shape: straight to slightly curved. Inclination: moderately prosoclin.
Endangered (B2ab(iii)+C2a(i) ver. 10.1). During a survey of a dozen isolated limestone hills of the State of Perlis in May 2011, only Bukit Kubu was found to support a living population ofPlectostoma kubuensis, consisting of several hundred individuals at the top of Bukit Kubu. Several very small populations (< 50 individuals) live at the other part of Bukit Kubu where the habitat is relatively more exposed and dry. Bukit Kubu is gazetted by Perlis State government as a recreation forest, and its surroundings consist of urban development and paddy fields.
Holotype: BOR 5648(1). Paratypes: BOR 5518(>25), BOR 5519(>10).
From the conchological point of view, the overall shape and size of Plectostoma kubuensis andPlectostoma kakiense is almost the same. Nevertheless, two inconspicuous but significant shell characters, namely, basal and parietal constriction teeth, mark the difference between these two species. Despite the similarity in shell form, there is a great genetic distance between the two species (16%). The species occur at two limestone hills that lie about 30 km apart. During our survey, we could not find either species at the six limestone hills that are located between these two hills. A single Plectostoma shell was recorded by Norhanis et al. (2010) from Pulau Dayang Bunting, Langkawi, and might belong to this species.
Type locality. Bukit Kubu (6°24'15"N, 100°8'37"E) (Figure 18C).
Distribution range. Endemic to the type locality.