Discartemon discadentus Siriboon & Panha

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 : 58-59

6. Discartemon discadentus Siriboon & Panha sp. n. Figs 2B, 4I, J, 12A, B, 17 F–I, 23, Table 1

Type material.

Holotype CUMZ 6244 (Fig. 4I). Measurement: shell height 4.5 mm, shell width 12.1 mm, and with 6 whorls. Paratypes: CUMZ 6003 (2 shells), 6209 (1 specimen in ethanol; Figs 2B, 12A, B, 17 F–I), 6258 (4 shells), NHMUK 20130672 (1 shell), and SMF (1 shell) from the type locality.

Type locality.

Wat Tam Yai, Thachana, Suratthani, Thailand (9°32'21.5"N, 99°11'29.4"E).

Diagnosis.

This new species can be distinguished from Discartemon discus and Discartemon sykesi by having transverse ridges that diminish below the periphery, and having an apertural dentition with five lamellae. In comparison, Discartemon sykesi has a smaller shell and Discartemon discus has a higher spire. The genitalia of Discartemon discus have a short penis, penial sheath and free oviduct, and long vagina while Discartemon discadentus sp. n. has a very long penis, penial sheath and free oviduct, and short vagina. Discartemon discadentus sp. n. differs from Discartemon nummus and Discartemon khaosokensis in having a larger shell with higher spire, in lacking a peripheral keel, and in usually having five apertural lamellae. The last whorl of Discartemon khaosokensis is rapidly expanded, while Discartemon nummus has a regularly expanded last whorl and Y-shaped parietal lamella. The genitalia of Discartemon discadentus sp. n. differ from those of Discartemon nummus in the long and slender penis, penial wall with reticulated folds, and long penial hooks located on conical penial papillae.

Description.

Shell. Shell flattened, white and translucent; whorls 6; spire only slightly elevated; suture distinct. Shell surface glossy with transverse ridges that diminish below periphery; varices present. Embryonic shell large, about 2½ whorls, with smooth surface; following whorls regularly coiled. Last whorl angular, intermediately expanded; umbilicus very wide and showing all preceding whorls. Aperture semi-ovate; peristome discontinuous, thickened and expanded. Apertural dentition usually with one strong parietal, one palatal, one small basal and one strong columellar lamella. A small supracolumellar lamella is sometimes present (Fig. 4I).

Genital organs. Atrium (at) short; penis (p) very long and slender. Penial sheath (ps) thin, extending about five-sixths of penis length; penial sheath retractor muscle very thin (psr), originating at genital orifice wall and inserting distally on penial sheath (Fig. 12A). Vas deferens (vd) passes through a very short part of penial sheath before entering into penis distally (Fig. 12B). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally corrugated (Fig. 17F). Penial wall with scattered and transparent penial hooks, about 6 hooks/200 µm 2 (Fig. 17G); hooks located on conical penial papillae (pp) separated by low reticulated folds. Penial hooks small (<0.03 mm in length), expanded at base, tips pointed and curved towards genital orifice (Fig. 17H).

Vagina (v) short, about one seventh of penis length. Gametolytic duct (gd) a long tube extending as far as albumin gland; gametolytic sac (gs) ovate. Proximal free oviduct (fo) convoluted and distally long and thick; oviduct (ov) enlarged and folded. Prostate gland inconspicuous and bound to oviduct. Talon (ta) small, short and club shaped. Hermaphroditic duct (hd) bearing long seminal vesicle (sv) about four times longer than the length from talon to branching point of seminal vesicle (Fig. 12A).

Vaginal wall generally with smooth surface of reticulated vaginal folds (Fig. 17I).

Etymology.

The specific epithet “discadentus” is derived from the Latin “discus” meaning “disc” and “dentatus” meaning “teeth”.

Distribution.

This species seems to be restricted to limestone areas in Suratthani Province, Thailand. Tam Khuha, Kanchanadit District, is an isolated limestone hill about 40 km southeast of the type locality and Wat Tam Wararam, Phanom District, is in the limestone mountains near Ratchaprapa Dam, about 70 km southwest of the type locality.

Remarks.

This species shows variation in shell size and the presence of the infrapalatal, upper palatal and supracolumellar lamellae. Some specimens from Phanom, Suratthani (CUMZ 3571, 3582) possess an upper palatal and supracolumellar lamella, and an infrapalatal lamella is present in one paratype shell (CUMZ 6003). Populations from Tam Khuha, Suratthani (CUMZ 6004) exhibit a relatively smaller shell size (width about 11 mm). This new species is apparently rare and only extensive searching revealed living animals.