to be added
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Holotype: RMNH 81805(1) (Seen). Paratypes: RMNH 81806(1) (Seen).
By Maassen (2001)
"Shell dextral, rather low conical, light brown, shining and transparent; with 5.5 distinctly canvex whorls, separated by a rather deep suture, and without a spiral striation. The body-whorl is wider than the penultimate whorl. Radial ribs rather thin, increasingly much higher and sharper on the lower whorls; on the penultimate whorl 4-6 ribs/mm, above the aperture with 4-6 ribs/mm. The constriction consists of a transversal thickening all around, and of a small and low, longitudinal tubercle at the angular edge, just entering the transversal thickening. The tuba is 0.5 whorl long. Umbilicus narrow. Peristome flaring, double, the inner peristome only slightly protruding from the outer. In the frontal view, aperture is situated at the right of the middle of the previous whorl and clearly turned upwards. Operculum corneous, with a distinct spiral ridge on its outer surface. Dimensions: Height 2.4-2.7 mm, Width 2.2-2.4 mm."
Plectostoma christae, together with Plectostoma dindingensis, Plectostoma mengaburensis,Plectostoma sinyumensis, Plectostoma umbilicatum, Plectostoma siphonostomum, andPlectostoma panhai represent a group of Plectostoma species that have a regulary coiled tuba (type 1 tuba). The species of this group occur only in Peninsular Malaysia and are genetically highly divergent (> 10% differences in COI) from the others (Table 4). All of the seven species are distributed allopatrically (Figure 18D).
We synonymised Plectostoma jensi with Plectostoma christae, both of which were described from the same locality. Maassen (2001) distinguished between them by the slight difference in umbilicus opening and aperture tilting. In the material at our disposal, we recognised that these differences are intrapopulational variation. All individuals share the same diagnostic shell characters as mentioned above. In addition to the morphological evidence, the genetic variation between individuals with different shell forms is smaller than our species delimitation threshold of 10%.
Two species of this group, namely Plectostoma christae and Plectostoma siphonostomum, have a wider distribution range than other species in this group. The two species occur parapatrically on the limestone hills in the centre of Peninsular Malaysia (Figure 18D). On the other hand, very little is known of the distribution range of Plectostoma panhai. Although this species was reported only once and only from the type locality, it might also occur at other limestone sites near the type locality. Plectostoma panhai is very similar to Plectostoma christae, but the two are separated by more than 150 km, and the limestone hills in between are occupied by three otherPlectostoma species. The disjunct distribution and its single constriction tooth support the decision that Plectostoma panhai is a distinct species from Plectostoma christae.
The remaining four species of this group, namely, Plectostoma sinyumensis, Plectostomamengaburensis, Plectostoma dindingensis, and Plectostoma umbilicatum, are site endemics, occurring at each of the four small limestone clusters in the centre of Peninsular Malaysia (Figure 18D). These clusters are each quite isolated, with no other limestone hills within a 20 km radius.
Although these four species occur in adjacent limestone limestone clusters, and they have similar shell shapes, their taxonomic status are clear. The COI sequence divergence between these species is larger than 13% and each of them has a set of diagnostic shell characters (Tables 3 and4). This may raise the question how each species evolved in each limestone cluster and how long these four species have been isolated. For example, a neighbouring species, Plectostomasalpidomon, has a similar distribution range as the former four species, but the morphological and genetic divergence is much smaller than in these four species. Presumably, the answer lies in the details of the geomorphological evolution of the limestone outcrops, which, however, remains largely unknown.
Type locality. Limestone hills, 16 km west of Gua Musang (4°54'46"N, 102°6'22"E).
Distribution range. Limestone hills along the 50 km Northeast transect between 4°38'51"N, 101°58'58"E and 5°0'13"N, 102°11'59"E (Figure 18D).