Plectostoma annandalei (Sykes, 1903)

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 : 67-68

Plectostoma annandalei (Sykes, 1903)

Opisthostoma annandalei Sykes, 1903: 198, Plate 20 - figures 4 & 5 (original description).

Opisthostoma annandalei Sykes, Laidlaw (1928: 36).

Opisthostoma annandalei Sykes, van Benthem Jutting (1952: 41).

Type material.

Not seen.

Diagnosis.

Shares with Plectostoma laidlawi , Plectostoma tenggekensis , and Plectostoma praeco the general shell form, in terms of spire and tuba shape, but differs by having slightly convex whorl periphery and straight ribs.

Description

(estimated from figure in Sykes 1903). Apex. Shape: moderately convex.

Spire. Height: 2 mm. Width: 1.3 mm. Number of whorls: 4 1/2. Apical spire shape: depressed conical. Basal spire shape: conical. Whorl periphery: moderately convex. Umbilicus: Open.

Constriction. Unknown.

Tuba. Coiling direction: type 2 and aperture visible in right lateral view. Tuba whorl length in proportion to spire last whorl: approximately the same as the spire’s last whorl length. 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. Unknown

Radial ribs. Rib density: 5 ribs per mm. Rib intensity: unknown. Shape: straight. Inclination: orthoclin.

Distribution.

Type locality. This species is only known from its type locality, Jalor (Biserat). The exact location was not described in the original publication of this species. From the collectors’ report (Annandale and Robinson 1913), we estimated the location and name of the limestone hill from a map in the report. This hill was named Bukit Bayu. Later, we estimated its coordinates from Google Earth (6°16'48"N, 101°13'35"E) (Figure 18B).

Distribution range. Unknown.

Conservation status.

Data Deficient.

Discussion.

This species has not been seen or collected after the original description. The type specimens cannot be located. Sykes (1903) mentioned that it is similar to Plectostoma laidlawi but he did not explain explicitly in what way. This is not unexpected, as Plectostoma laidlawi was the only one Plectostoma known from Peninsular Malaysia at the time, although more than 20 Plectostoma species had already been described from Borneo. Thus, one can assume that the statement made by Sykes was based on the geographical proximity.

Interestingly, Plectostoma panhai was described from a limestone hill located just 8 km from the location where Plectostoma annandalei was found. In addition to the geographical proximity, the two neighbouring species are similar in several shell characteristics. The shell spires are very similar in terms of number of whorls, overall shape and size. On the other hand, the main difference between these two species is the tuba coiling direction (type 1 vs. type 2).