Rhiostoma marioni (Ancey, 1898)

Tongkerd, Piyoros, Tumpeesuwan, Sakboworn, Inkhavilay, Khamla, Prasankok, Pongpun, Jeratthitikul, Ekgachai, Panha, Somsak & Sutcharit, Chirasak, 2023, Systematic revision of the snorkel snail genus Rhiostoma Benson, 1860 (Gastropoda, Caenogastropoda, Cyclophoridae) with descriptions of new species, ZooKeys 1142, pp. 1-144 : 1

7. Rhiostoma marioni (Ancey, 1898)

Figs 18 , 24B-F

Pterocyclos marioni (in part) Ancey, 1898: 137, pl. 9, fig. f. Type locality: Luang-prabang, Laos and Mont Hou, Tonkin. Fischer and Dautzenberg 1904: 430. Wood and Gallichan 2008: 64, pl. 25, fig. 3 (syntype).

Pterocyclus marioni -Kobelt 1911: 757.

Type material.

The figured specimens in Ancey (1898: pl. 9, fig. f) could not be located.

Other material examined.

Laos: Wat Non Hinhare, Feuang District, Vientiane Province: CUMZ 10210 (7 shells; Fig. 24C-F )

Diagnosis.

Shell medium, thick and depressed shell. Detached whorl long, ~2 × longer than aperture width, curved, slightly descending. Breathing device tubular and straight. Shell colour uniformly brownish or with irregular blotches.

Differential diagnosis.

Rhiostoma marioni has been considered a junior synonym of R. housei from Thailand (Kobelt 1911). However, this species differs from R. housei in having a monochrome brownish shell colour, long detached whorl, and long straight tubular breathing device. In comparison, R. housei tends to have a brownish zigzag pattern, medium detached-whorl length, and a curved breathing tube.

Description.

Shell. Shell medium, cW 17.1-20.5 mm, cH 9.1-10.6 mm, thick, and sub-discoidal to discoidal shape; detached-whorl length 14.0-20.5 mm. Apex acute; spire slightly elevated. Whorls 5 to 6, convex, increasing regularly; suture wide and deep; last whorl rounded and stout. Shell surface with fine growth lines. Periostracum thin corneous, and transparent. Shell colour uniformly brownish to dark brown (rarely with brownish blotches); ventral shell surface with paler colour; with narrow and dark spiral band on periphery. Detached whorl long, ~ 2 × longer than apertural width, curved and descending. Peristome circular and double; lip thickened, expanded and multi-layered. Aperture opened sub-ventrally. Breathing device tubular and its tip often attached to preceding whorl; outer lip forming a long, straight, closed tube, located just behind apertural lip; inner lip with small hole inside aperture. Umbilicus widely opened and deep. Operculum calcareous, cup-shaped, and multispiral (Fig. 24B-F ).

Distribution.

This species is currently known only from Vientiane Province, Laos (Fig. 18 ).

Remarks.

This species was described based on two specimen lots from "Luang-prabang, Laos" and "Mont Hou, Tonkin" [probably in the area of Muang Khua and Yot Ou Districts, Phongsaly Province, Laos (see Pavie 1903: pl. 1)]. The original description includes an illustration of a single specimen and one set of measurements ( Ancey 1898). Unfortunately, this figured specimen could not be located from the RBINS or NMW collection. The specimen lot NMW 1955.158.24090 consisting of a single specimen (with operculum) that has Ancey’s type label and collection locality “Luang-prabang” was considered the possible holotype by Wood and Gallichan (2008: pl. 25, figs 3, iv). However, this surviving specimen looks different from the actual illustrated specimen in the original publication. This specimen has a flattened shell, short detached whorl, and notch-shaped breathing device projecting anteriorly (Fig. 24A ). Although the operculum is not clearly mentioned in the original description, the operculum accompanied by this specimen lot (NMW 1955.158.24090) is dark corneous, plate-shaped and multispiral with elevated lamellae (Fig. 24A ). It is distinct from the typical operculum of Rhiostoma (Fig. 2 ), and more closely resembling the Ptychopoma or Scabrina. This specimen is contradicting to Ancey’s concept of " Pterocyclos marioni ", whereas the species description agrees well with the specimen illustration ( Ancey 1898: pl. 9, fig. f). Therefore, we consider specimen NMW 1955.158.24090 (Fig. 24A ; Inkhavilay et al. 2019: fig. 11a; Do et al. 2020b: fig. 1d) as a distinct species. We have excluded that specimen from our attention in the type series.

This redescription is based on the recently collected specimens from Feuang District, Vientiane Province, Laos. Before establishing the French Protectorate of Laos in 1893 ( Pavie 1903: pl. 6; Tips 1999: 131), the historical name "Luang Prabang" generally referred to the city or the Kingdom Luang Prabang (during the 18th to 19th century). This name encompasses a wide geographical area, mainly present-day northern Laos: Bokeo, Houa Phanh, Luang Namtha, Luang Prabang, Oudomxay, Phongsaly and Vientiane provinces. Therefore, this newly collected specimen from Vientiane Province is considered the topotypic specimen of this species. This specimen lot possesses a long detached whorl and a long tubular-shaped breathing device identical to the unique characteristic of R. marioni ; it agrees well with the illustration (Fig. 24B ) and the species description.

The previous records under the name Rhiostoma " Rhiostoma marioni " from Lai Chu, Son La, Dien Bien and Lau Cai provinces, North Vietnam, as mentioned by Do et al. (2020b), should be referred to another species (see under R. abletti ).

Figures

Figure 2. Schematic of shell morphology and terminology of the Rhiostoma A apical view with shell terminology and measurements of coiled-whorl width and detached-whorl length B operculum showing side and cross-section views C apertural views showing various shell shapes as recognised in this revision: flattened, discoidal, sub-discoidal and heliciform.

Figure 18. Distribution map of Rhiostoma housei - species group.

Figure 24. A specimen mentioned as being the syntype of " Pterocyclos marioni Ancey, 1898 " NMW 1955.158.24090. B-F shell of Rhiostoma marioni B published figure of the type specimen (after Ancey 1898: pl. 9, fig. f.) and C-F specimens CUMZ 10210 from Feuang, Vientiane, Laos.