Discartemon triancus 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 : 84-85

26. Discartemon triancus Siriboon & Panha sp. n. Figs 3F, 10C, D, 16 D–F, 21 G–L, 22H, 23, Table 3

Type material.

Holotype CUMZ 6254 (Fig. 10C). Measurement: shell height 4.6 mm, shell width 7.3 mm, and with 6 whorls. Paratypes: CUMZ 6032 (2 shells), 6236 (6 specimens in ethanol), and NHMUK 20130682 (2 shells) from the type locality.

Type locality.

Gunung Kilian, Perlis, Malaysia, 6°34'8.0"N, 100°11'44.4"E.

Diagnosis.

This new species is superficially similar to Discartemon roebeleni and Discartemon kotanensis sp. n., but the distinguishing characters are the smaller shell, lower spire, angular last whorl, very long penis and free oviduct, short seminal vesicle, and penial hooks with elongated bases. Discartemon triancus sp. n. can be distinguished from Discartemon megalostraka sp. n. by having a smaller shell, lower spire, four apertural lamellae, a longer penis, short free oviduct, and slender penial hooks with elongated bases. Discartemon triancus sp. n. differs from Discartemon conicus sp. n. in having a lower spire with shallow suture, transverse ridges, in lacking a sinulus, and in having four apertural lamellae.

Description.

Shell. Shell globose-heliciform, white and translucent; whorls 5½– 6, spire only slightly convex, with distinct suture. Shell surface glossy with transverse ridges that diminish below the periphery; varices present. Embryonic shell large, about 2½ whorls, with a smooth surface; following whorls regularly coiled. Last whorl angular, regularly expanded; umbilicus widely open and deep. Aperture subcircular; peristome discontinuous, thin and expanded. Apertural dentition of one parietal, one palatal, one small basal and one columellar lamella (Fig. 10C).

Radula. Each row consists of 27-43 teeth with formula (13-21)-1-(13-21). The central tooth is very small with pointed cusp. Lateral and marginal teeth are undifferentiated, unicuspid and lanceolate. Latero-marginal teeth gradually reduce in size, with outermost teeth much smaller and shorter than inner teeth (Fig. 22H).

Genital organs. Atrium (at) very short. Penis (p) extremely thin, long; becoming enlarged distally. Penial sheath (ps) thin, extending about half of penis length. Penial sheath retractor muscle very thin (psr), originating at atrium and inserting distally on penial sheath (Fig. 16D). Vas deferens (vd) passes a very short distance through penial sheath before entering into penis distally (Fig. 16E). Penial retractor muscle (pr) thin and very long, inserting at penis and vas deferens junction.

Internal wall of atrium generally smooth with pores (Fig. 21G). Penial wall with scattered and transparent penial hooks, about 11 hooks/200 µm 2 (Fig. 21H). Hooks located on penial wall. Penial hooks small (<0.04 mm in length), short, with strongly elongated bases, tips pointed, and curved towards genital orifice (Fig. 21 I–K).

Vagina (v) short. Gametolytic duct (gd) a long and slender tube extending as far as albumin gland; gametolytic sac (gs) ovate. Free oviduct (fo) with almost same diameter as vagina and about twice as long as vagina. Oviduct (ov) enlarged and folded; prostate gland inconspicuous. Talon (ta) small, short and slender. Hermaphroditic duct (hd) bearing a short seminal vesicle (sv) nearly equal to the length from talon to branching point of seminal vesicle (Fig. 16F).

Vaginal wall with longitudinal vaginal folds (Fig. 21L).

Etymology.

The specific epithet “triancus” is derived from the Latin “triangulum” meaning “triangle” and “uncus” meaning “hook”.

Distribution.

Known only from the type locality.

Remarks.

Material from Gunung Kilian, Perlis, Malaysia was firstly identified as Discartemon roebeleni ( Möllendorff, 1894) by Benthem Jutting (1954), without any anatomical comparison. However, clear anatomical differences between this new species and Discartemon roebeleni , so it is considered a new species.