Plectostoma senex (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 : 44-45

Plectostoma senex (van Benthem Jutting, 1952) Figures 17H, 30, Appendix 7

Opisthostoma senex van Benthem Jutting, 1952: 47, figure 24 (original description).

Type material.

Holotype: ZMA 136052(1) (Seen). Paratypes: ZMA 136053(>10) (Seen).

Other examined materials.

RMNH 44723(5), BOR 460(1), BOR 5575(3), BOR 5603(2), BOR 5628(>100), BOR 5631(>10), V 5117(9).

Diagnosis.

Shares with Plectostoma sciaphilum and Plectostoma turriforme the tuba form, but differs by having two knob-shaped constriction teeth and fewer than 6 whorls.

Description.

Apex. Shape: distinctly convex.

Spire. Height: 2.6-3.1 mm. Width: 1.6-1.7 mm. Number of whorls: 5 1/8-5 5/8. Apical spire shape: oblong conical. Basal spire shape: ovoid. Whorl periphery: moderately to distinctly convex. Umbilicus: completely closed by tuba.

Constriction. Parietal teeth: two. Basal teeth: one transverse tooth.

Tuba. Coiling direction: type 3 and aperture visible in left lateral view. Tuba whorl length in proportion to spire last whorl: ca. 3/8. 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: 4 ribs per mm. Rib intensity: thin. Shape: slightly curved. Inclination: orthoclin.

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. In addition to the type locality, this species had been recorded from Bukit Panching (Figure 18A).

Conservation status.

Critically Endangered (B2ab(iii)+C2a(i) ver. 10.1). This species is known from two locations. One of these, Bukit Panching, does not exist anymore. Two intensive surveys at the other location, Bukit Charas, were conducted in Feb. 2010 and May 2011. Two living populations with fewer than 50 individuals were found at wet staglamites near the cave. No other living population was found elsewhere on Bukit Charas. The status of this species in a previous assessment (IUCN redlist) was: vulnerable D2 ver. 3.1 ( Clements 2009b).

Discussion.

See discussion under Plectostoma turriforme .