Pseudoplecta bijuga, (STOLICZKA, 1873)

Jirapatrasilp, Parin, Tongkerd, Piyoros, Jeratthitikul, Ekgachai, Liew, Thor-Seng, Pholyotha, Arthit, Sutcharit, Chirasak & Panha, Somsak, 2021, Molecular phylogeny of the limacoid snail family Dyakiidae in Southeast Asia, with the description of a new genus and species, Zoological Journal of the Linnean Society 193 : -

PSEUDOPLECTA BIJUGA ( STOLICZKA, 1873)

( FIGS 6C–F, 7C, 8C, 9)

Z o o b a n k r e g i s t r a t i o n: u r n: l s i d: z o o b a n k. org:act: 05AD59EE-788B-4597-BEA9-704F892F2184

Synonymy

Rotula bijuga – Stoliczka, 1873: 14–16, pl. 1, figs 4–7, pl. 2, figs 16–18. Type locality: Penang.

Macrochlamys pataniensis – Morgan, 1885: 376, pl. 5, fig. 5. Type locality: Naxa dans le district de Rhaman, royaume de Patani .

Rotularia bijuga – Morgan, 1885: 378.

Nanina (Rotula) bijuga – Tryon, 1886: 22, pl. 4, figs 56–59.

Euplecta bijuga – Möllendorff, 1891: 332. – Collinge, 1903: 208.

Euplecta pataniensis – Möllendorff, 1891: 333.

Otesia bijuga convexospira – Möllendorff, 1902: 137. Type locality: Kelantan, Ostküste der Halbinsel Malacca. New synonym .

Pseudoplecta bijuga – Laidlaw, 1932: 91–93, figs 7, 8. – Laidlaw, 1933: 227, 228. – Schileyko, 2003: 1363, 1364, fig. 1779. – Schileyko, 2007: 2112, fig. 1779a. – Foon et al., 2017: 65, fig. 25b.

Pseudoplecta bijuga bijuga – Maassen, 2001: 104.

Type material examined

Possible syntypes of Rotula bijuga Stoliczka, 1873 , NHMUK 20200203 (two shells; Fig. 6C). Syntype o f M a c r o c h l a m y s p a t a n i e n s i s M o r g a n, 1 8 8 5, MNHN-IM-2000-34171 (three shells; Fig. 6D).

Material examined

Berenchang Road, Cameron Highlands 4500 ft., NHMUK 1933.3.24.2-3 (two shells; labelled as paratypes of Pseudoplecta cameroni n. gen.? n. sp. MSS; Fig. 6E). Gua Matu Madu , Gua Musang, Kelantan, Malaysia: CUMZ 5150 (three specimens in ethanol; Figs 7C, 8C, 9), CUMZ 5151 (one shell; Fig. 6F) .

Diagnosis

Shell small, slightly thin, depressed to ovate conic, with fine radial ridges. Penis long, with penial verge inside. Gametolytic organ located close to terminal part of amatorial organ; amatorial organ gland and duct absent; amatorial organ papilla without corneous spike.

Description

Shell ( Fig. 6C–F): Shell small (width ≤ 17.8 mm, height ≤ 11.3 mm), dextral, slightly thin, strongly depressed to ovate conic and moderately translucent. Whorls six to eight, little convex and increasing regularly; suture wide and shallow. Shell colour corneous; upper and below periphery with indistinct colour. Apex acute; embryonic shell with nearly smooth surface and ~1½ whorls. Following whorls with series of thin nodules arranged on fine radial ridges; upper periphery with strong radial ridges and weaker below. Last whorl angulated, with strong peripheral keel. Aperture subovate and oblique; lip simple and slightly thickened. Columella little dilated; parietal callus transparent. Umbilicus perforate and deep.

Genitalia ( Fig. 9): Atrium short. Penis close to atrium enlarged, cylindrical and gradually tapering to small tube. Epiphallus long, almost the same length as penis and approximately similar diameter to penis. Penial retractor muscle with thick band, very short compared with penis, and attached between penis and epiphallus. Flagellum absent. Vas deferens relatively smaller diameter than epiphallus and connected at tip of epiphallus. Internal wall of penis with large and corrugated penial pilasters surrounding penial verge and nearly smooth surface close to atrium; penial verge small and low conical shape.

Gametolytic organ (duct and sac) close to terminal part of amatorial organ. Gametolytic duct cylindrical tube; gametolytic sac enlarged at the end. Amatorial organ well-developed cylinder and attached to atrium; region close to atrium swollen, round (with amatorial organ papilla inside); region close to gametolytic sac enlarged cylinder, with thick retractor muscle. Amatorial organ gland and duct absent. Internal wall sculpture of amatorial organ: close to atrium consists of irregular and smooth-surfaced amatorial organ pilasters, gradually modified to corrugated and nearly smooth wall. Amatorial organ papilla obtuse in shape, with corrugated surface; corneous spike absent.

Vagina about two-thirds the length of penis and cylindrical. Oviduct with enlarged lobules; prostate gland bound to oviduct. Internal wall of vagina mostly sculptured with irregular, undulated pilasters of similar size and arranged in longitudinal rows.

Radula ( Fig. 7C): Teeth arranged in wide-angled U-shaped rows, with ~190 (93-(22–20)-1-(21–23)-98). Central tooth symmetric tricuspid with triangularshaped mesocone; ectocones small and dull cusp. Lateral teeth asymmetric tricuspid, triangular in shape and inclined towards central tooth; endocone and ectocone nearly wanting. Marginal teeth start from tooth 20–23, elongate, obliquely bicuspid and sword shaped; endocone large and pointed cusp; ectocone small and pointed cusp. Outermost teeth shorter than inner teeth and ectocone small.

External features ( Fig. 8C): Living snail typical dyakiid form; body reticulated and pale orange. Tentacles long and brown. Dorsal part darker, with black stripes running from base of tentacles to pneumostome. Foot aulacopod and unipartite; foot sole brownish to orange and unspotted.

Remarks

This species was originally described from Penang [ Island, Malaysia], with detailed description and illustration by Stoliczka (1873). Later, de Morgan (1885) described Macrochlamys pataniensis from ‘Naxa dans le district de Rhaman, royaume de Patani’ [Raman District, Pattani Province, Thailand] as a distinct species, without any detail on internal anatomy. These two nominal species have been treated as distinct ever since, and their generic placements were hypothetical. Later, Laidlaw (1932) could not affiliate Stoliczka’s species to any existing genus and therefore introduced the genus Pseudoplecta , and Laidlaw (1933) treated M. pataniensis as a junior synonym to this species. Here, we synonymize M. pataniensis with Pseudoplecta bijuga because this species shows a large variation in spire height [see Stoliczka (1873): pl. 1, figs 4–7], and M. pataniensis is considered to be within the range of this variation. In addition, the shell-based description of Otesia bijuga convexospira by von Möllendorff (1902) is also contained within the variation, and the type locality was within the distributional range of this species. Hence, we treat this nominal taxon as a synonym of Pseudoplecta bijuga .