Plectostoma charasense (Tomlin, 1948)

Liew, Thor-Seng, Vermeulen, Jaap Jan, Marzuki, Mohammad Effendi bin & Schilthuizen, Menno, 2014, A cybertaxonomic revision of the micro-landsnail genus Plectostoma Adam (Mollusca, Caenogastropoda, Diplommatinidae), from Peninsular Malaysia, Sumatra and Indochina, ZooKeys 393, pp. 1-107 : 64-65

Plectostoma charasense (Tomlin, 1948) Figure 44, Appendix 14

Opisthostoma charasense Tomlin, 1948: 225, Plate 2 - figure 4 (original description).

Opisthostoma charasense Tomlin, van Benthem Jutting (1952: 42).

Type material.

Holotype: BNHM 1948.10.2.2(1) (Seen).

Other examined materials.

ZMA 162063(>10), ZMA 162064(>10).

Diagnosis.

Shares with Plectostoma kitteli , Plectostoma tohchinyawi , and Plectostoma klongsangensis the general shell form, in terms of apex, spire, and tuba shape, but differs by lacking thick spiral lines and by having two basal constriction teeth.

Description.

Apex. Shape: distinctly convex.

Spire. Height: 1.9-2.0 mm. Width: 1.3-1.4 mm. Number of whorls: 4 1/8. Apical spire shape: oblong conical. Basal spire shape: conical. Whorl periphery: distinctly convex. Umbilicus: open.

Constriction. Parietal teeth: two. Basal teeth: two.

Tuba. Coiling direction: type 2 and aperture visible from right lateral view. Tuba whorl length in proportion to spire last whorl: ca. 7/8. Proportion of tuba that attaches to spire: less than 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: absent. Thin lines: present.

Radial ribs. Rib density: 4 ribs per mm. Rib intensity: thick. Shape: slightly curved to single-humped. Inclination: moderately prosoclin.

Distribution.

Type locality. Gua Charas, limestone hill near Sungai Lembing in the state of Pahang, Malaysia (3°54'27"N, 103°8'47"E).

Distribution range. Until now, this species has been recorded from two neighbouring limestone hills, namely, Gua Charas and Bukit Panching. However, the whole hill of Bukit Panching has been lost to quarrying (Figure 18B).

Conservation status.

Critically Endangered (B2ab(iii)+C2a(i) ver. 10.1). Gua Charas might support a viable population. However, the hill is currently surrounded by oil palm plantation with a very narrow forested buffer zone. No living individuals or fresh dead shells were found at Gua Charas after exhaustive search during several sampling trips in 2010 and 2011.

Discussion.

The taxonomic status of this species is stable. Although the two neighbouring species, Plectostoma tohchinyawi and Plectostoma praeco , are similar to Plectostoma charasense , there are a few key shell characters that separate Plectostoma charasense from the others. See discussion under Plectostoma tohchinyawi for more discussion about the biogeography of this species.