Plectostoma umbilicatum (van Benthem Jutting, 1952)

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 : 33-34

Plectostoma umbilicatum (van Benthem Jutting, 1952) Figure 17B, 22, Appendix 8

Opisthostoma umbilicatum van Benthem Jutting, 1952: 49, figure 25 (original description).

Opisthostoma umbilicatum van Benthem Jutting, van Benthem Jutting (1961: 39).

Opisthostoma umbilicatum van Benthem Jutting, Clements (2007: 74).

Opisthostoma umbilicatum van Benthem Jutting, Clements et al. (2008: 2760).

Type material.

Holotype: ZMA 136070(1) (Seen). Paratypes: ZMA 136071(8) (Seen).

Other examined materials.

BOR 5503(>10), BOR 5625(>25).

Diagnosis.

Shares with Plectostoma dindingensis , Plectostoma mengaburensis , and Plectostoma panhai the tuba form, but differs by having an oblong conical apical spire.

Description.

Apex. Shape: moderately convex.

Spire. Height: 2.0-2.3 mm. Width: 1.3-1.5 mm. Number of whorls: 4 5/8-4 3/8. Apical spire shape: oblong conical. Basal spire shape: ovoid. Whorl periphery: distinctly convex. Umbilicus: completely open or partially closed by tuba.

Constriction. Parietal teeth: none. Basal teeth: none.

Tuba. Coiling direction: type 1 and aperture visible from back view. Tuba whorl length in proportion to spire last whorl: ca. 1/2-3/4. Proportion of tuba that attaches to spire: whole.

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: 6-9 ribs per mm. Rib intensity: thin. Shape: straight. Inclination: orthoclin.

Distribution.

Type locality. Limestone hill cluster named Kota Tongkat (3°53'28"N, 102°28'23"E).

Distribution range. It is only known from the type locality. All other adjacent limestone outcrops have been sampled, but only other Plectostoma species were found (Figure 18D).

Conservation status.

Critically Endangered (B2ab(iii)+C2a(i) ver. 10.1). The Kota Tongkat limestone cluster is surrounded by oil palm plantation and heavily degraded forest. This species is only known from this limestone cluster. Recent soil samplings have not revealed any recent dead shells (Clements et al. 2008). Neverthelss, we found a living population with fewer than 100 individuals at a wet stalagmite of the entrance of one the caves during an intensive survey in May 2011. During that survey, we noticed that all other rock surfaces of the limestone outcrops in Kota Tongkat were very dry. Thus, the recorded and other unknown living populations are at risk of extinction because a long drought might wipe them out.

Discussion.

See discussion under Plectostoma christae .