Plectostoma sinyumensis (Maassen, 2001)

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 : 32-33

Plectostoma sinyumensis (Maassen, 2001) Figures 17L, 21, Appendix 7

Opisthostoma sinyumensis Maassen, 2001: 52, figures 1, 6 & 7 (original description).

Opisthostoma sinyumensis Maassen, Clements (2007: 74).

Opisthostoma sinyumensis Maassen, Clements et al. (2008: 2760).

Opisthostoma sinyumensis Maassen, Webster et al. (2012: 628).

Type material.

Holotype: ZMA 138439(1) (Seen). Paratypes: ZMA 138440(>25) (Seen), RMNH 81804(2) (Seen).

Other examined materials.

BOR 462(5), BOR 5537(>10), BOR 5623(>50).

Diagnosis.

Shares with Plectostoma dindingensis , Plectostoma mengaburensis , Plectostoma christae , and Plectostoma panhai the general shell form, in terms of apex, apical spire and tuba, but differs by having an ellipsoid basal spire.

Description.

Apex. Shape: moderately convex.

Spire. Height: 1.6-1.9 mm. Width: 1.2-1.3 mm. Number of whorls: 3-3 3/4. Apical spire shape: depressed conical. Basal spire shape: ellipsoid. Whorl periphery: distinctly convex. Umbilicus: almost completely closed by tuba.

Constriction. Parietal teeth: none. 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. 5/8-3/4. 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: absent. Thin lines: present.

Radial ribs. Rib density: 9-10 ribs per mm. Rib intensity: thin. Shape: straight. Inclination: orthoclin.

Distribution.

Type locality. Gunung Senyum, Pahang (3°42'35"N, 102°26'3"E).

Distribution range. In addition to the type locality, this species also was found at the Jebak Puyuh limestone outcrop, which lies about 1 km east of Gunung Senyum (Figure 18D). BOR 462 was collected in Pulau Singa Besar, which is located about 300 km from type locality. The reliability of the collection data is dubious (the same was found for BOR 463 of Plectostoma relauensis ).

Conservation status.

Near Threatened. There are four limestone hills in this cluster. Gunung Senyum and Jebak Puyuh are the larger among these four hills. The former is gazetted as recreation forest but the latter has been at risk of destruction. Jebak Puyuh had been earmarked for quarrying several years ago, but the plan has been abandoned. Plectostoma sinyumensis has been recorded from these two hills, but its status at the two smaller hills remains unknown. In a survey in July 2010, a living population was recorded at Jebak Puyuh, in an enclosed humid sinkhole. Intensive surveying on the limestone rock faces of Gunung Senyum and other parts of Jebak Puyuh has failed to retrieve any additional living individuals, probably because most of the rock faces were very dry.

Discussion.

See discussion under Plectostoma christae .