10. Discartemon expandus Siriboon & Panha sp. n. Figs 5G, H, 23, Table 1

Type material.

Holotype CUMZ 6248 (Fig. 5G). Measurement: shell height 3.8 mm, shell width 8.3 mm, and with 5½ whorls. Paratypes: CUMZ 3664 (10 shells), NHMUK 20130676 (2 shells), and SMF (2 shells) from the type locality.

Type locality.

Klong Hoy, Suratthani, Thailand, 8°57'18.1"N, 98°48'30.7"E.

Diagnosis.

Discartemon expandus sp. n. differs from Discartemon discus and Discartemon sykesi in its smaller shell with transverse ridges, intermediately expanded last whorl, and widely expanded peristome. In addition, a sinulus is absent in Discartemon discus . Discartemon expandus sp. n. can be distinguished from Discartemon khaosokensis by having a flattened spire, an angular and intermediately expanded last whorl, and a widely expanded peristome. Compared with Discartemon discadentus sp. n. and Discartemon discamaximus sp. n., Discartemon expandus sp. n. has a smaller shell with strong transverse ridges, a sinulus and a widely expanded peristome. Moreover, Discartemon discadentus sp. n. has five apertural lamellae, and Discartemon discamaximus sp. n. has a rapidly expanded last whorl.

Description.

Shell. Shell flattened, white and semi-transparent; whorls 5½– 6, spire flattened with a distinct suture. Shell surface glossy with transverse ridges that diminish below periphery; varices present. Embryonic shell large, about 2 –2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl angular, intermediately expanded; umbilicus very wide, deep and showing all preceding whorls. Aperture semi-ovate with narrow sinulus; peristome discontinuous, thin and widely expanded. Apertural dentition of only one parietal lamella (Fig. 5G).

Etymology.

The specific epithet “expandus” is derived from the Latin “expandi” meaning “expand” . It refers to the expanded peristome of this species.

Distribution.

The species is known only from the type locality and extensive searching revealed no living examples.

Remarks .

Some variation has been observed in the spire, which is slightly convex rather than flattened in some specimens, and in the distinctness of the suture.