Discartemon nummus (Laidlaw, 1929)

Siriboon, Thanit, Sutcharit, Chirasak, Naggs, Fred, Rowson, Ben & Panha, Somsak, 2014, Revision of the carnivorous snail genus Discartemon Pfeiffer, 1856, with description of twelve new species (Pulmonata, Streptaxidae), ZooKeys 401, pp. 45-107 : 56-57

4. Discartemon nummus (Laidlaw, 1929) Figs 2A, 4G, 11D, E, 17 A–E, 22B, 23, Table 1

Odontartemon (Discartemon) nummus Laidlaw, 1929: 259, 260, fig. 1. Type locality: Tale Sap, Singgora. Laidlaw 1933: 234.

Discartemon nummus - Benthem Jutting 1954: 87, 88. Benthem Jutting 1959: 168. Richardson 1988: 183.

Material examined.

Khao Ok Thalu, Phatthalung, Thailand (7°37'39.1"N, 100°5'19.1"E): CUMZ 3594 (24 shells; Fig. 4G) and 6208 (12 specimens in ethanol; Figs 2A, 11D, E, 17 A–E, 22B).

Description.

Shell. Shell flattened, white and semi-transparent; whorls 5½, spire flattened with distinct suture. Shell surface glossy with thin transverse ridges near suture and varices present. Embryonic shell about 2½ whorls; following whorls regularly coiled. Last whorl angular with strong peripheral keel, regularly expanded; umbilicus very wide and showing all preceding whorls. Aperture triangular with sinulus; peristome continuous, thickened, expanded and reflected. Apertural dentition with a Y-shaped parietal lamella adjoining at sinulus (Fig. 4G).

Radula. Each row consists of 39-41 teeth with formula (19-20)-1-(19-20). Central tooth very small and triangular with pointed cusp. Lateral and marginal teeth undifferentiated, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth smaller and shorter than inner teeth (Fig. 22B).

Genital organs. Atrium (at) long. Proximal penis (p) long, slender; distal penis globularly enlarged. Penial sheath (ps) thin, extending about two-thirds of penis length; penial sheath retractor muscle very thin (psr), originating at genital orifice wall and inserting distally on penial sheath (Fig. 11D). Vas deferens (vd) passes through about one-fifth of penial sheath length before entering into penis distally (Fig. 11E). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally smooth (Fig. 17A); penial wall with scattered, short and transparent penial hooks, about 5 hooks/200 µm 2 (Fig. 17B); hooks located on round-ovate penial papilla. Penial hooks of small size (<0.04 mm in length), slightly expanded at base, tip obtuse and directed towards genital orifice (Fig. 17C, D).

Vagina (v) short and stout, about half of penis length. Gametolytic duct (gd) a long tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) long and thick; oviduct (ov) enlarged and folded; prostate gland inconspicuous and bound to oviduct. Talon (ta) small, very short and club shaped. Hermaphroditic duct (hd) bearing long seminal vesicle (sv) about one and half times longer than the length from talon to branching point of seminal vesicle (Fig. 11D).

Vaginal wall generally with smooth surface of longitudinal vaginal folds (Fig. 17E).

Remarks.

Discartemon nummus was described from Tale Sap (= Lake or Lagoon), Singgora (= Songkhla). In this study, living snails were found at an isolated limestone hill near the lake in Phatthalung, about 60 km north of the type locality.

Having the smallest shell size clearly discriminates Discartemon nummus from all congeners. It is similar to Discartemon discus , which has a larger shell and a peripheral keel, lacks a sinulus, and has a semi-ovate aperture with a straight parietal lamella. Discartemon nummus can be distinguished from Discartemon khaosokensis in having a flattened spire, the last whorl regularly expanded, a triangular aperture, and a Y-shaped parietal lamella.