Plectostoma tonkinianum (Dautzenberg & Fischer, 1905)

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 : 40-41

Plectostoma tonkinianum (Dautzenberg & Fischer, 1905) Figure 27, Appendix 8

Opisthostoma tonkinianum Dautzenberg & Fischer, 1905: 444, plate 10 - figure 5, 6 and 7 (original description).

Opisthostoma tonkinianum Dautzenberg & Fischer, Saurin (1953: 134).

Opisthostoma tonkinianum Dautzenberg & Fischer, van Benthem Jutting (1962: 12).

Opisthostoma tonkinianum Dautzenberg & Fischer, Fischer (1963: 34).

Type material.

Not seen.

Other examined materials.

ZMA 162136(1), V 10000(>25), V 10010(>25), V 10022(>25), V 11243(>25), V 11270(>25), V 11432(3), V 11502(>50), V 7937(5), V 9940(>10), V 9957(>25).

Diagnosis.

Shares with Plectostoma whitteni the general shell form, in terms of apex, spire and tuba, but differs by having two lamella-shaped constriction teeth and a higher spire (> 3 mm).

Description.

Apex. Shape: distinctly convex.

Spire. Height: 3.1-3.8 mm. Width: 2.4-2.6 mm. Number of whorls: 4 3/4-4 7/8. Apical spire shape: depressed conical. Basal spire shape: ovoid. Whorl periphery: distinctly convex. Umbilicus: completely closed by tuba.

Constriction. Parietal teeth: two lamellae. Basal teeth: none.

Tuba. Coiling direction: type 3 and aperture visible in left lateral view. Tuba whorl length in proportion to spire last whorl: ca. 5/8. Proportion of tuba that attaches to spire: whole.

Aperture and peristome. Peristome: simple peristome.

Spiral lines. Thick lines: absent. Thin lines: present.

Radial ribs. Rib density: 6-7 ribs per mm. Rib intensity: thin. Shape: slightly curved to single-humped. Inclination: orthoclin.

Distribution.

Type locality. “Tonkin” ( Dautzenberg and Fischer 1905). The exact locality should be the limestone hill near Chau Dao.

Distribution range. Limestone hills in the Province of Kien Giang (Figure 18A).

Conservation status.

Data deficient. Some recent dead shells have been collected but no information is available on the habitat and population status.

Discussion.

In recent years, this species has been recorded at several limestone hills in the vicinity of Ha Tien, which is a popular tourism site. Although the type specimen cannot be located and the exact type locality cannot be determined, the shell morphology of the recent material fits well with the description in the original publication.

In the original publication, Dautzenberg and Fischer (1905) mentioned that this species was collected by M. Mansuy from Tonkin (French protectorate), which was a large area that included part of Southern China, and the Northern parts of Laos and Vietnam. Dautzenberg and Fischer (1905) further provided a list of locations where most of the snails were collected. One of these was, Chau Doc, which is located about 70 km from Ha Tien. Furthermore, a recent intensive land snail survey in the Northern Provinces of Laos, namely, Hua Phan and Luang Prabang, did not discover any Plectostoma species (Muratov and Abdou 2006). Hence, Plectostoma tonkinianum probably occurs in the small limestone hill cluster in the coastal area of the Southern part of Vietnam and neighboring Cambodia.