Rhiostoma simplicilabre Pfeiffer, 1862

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

6. Rhiostoma simplicilabre Pfeiffer, 1862

Figs 18 , 23

Rhiostoma simplicilabre Pfeiffer, 1862a: 115, pl. 12, fig. 7. Type locality: Camboja [Cambodia]. Pfeiffer 1865: 39. Fischer 1891: 101. Kobelt and Möllendorff 1897: 115. Kobelt 1902: 178, 179. Fischer and Dautzenberg 1904: 427. Kobelt 1911: 756, 757, pl. 110, figs 5-7, pl. 113, fig. 3. Sutcharit et al. 2019: 48, 49, fig. 11f, g.

Pterocyclos simplicilabris - Reeve 1863: Pterocyclos pl. 4, species 20.

Pterocyclus [sic] ( Rhiostoma ) Pterocyclos simplicilabris - Nevill 1878: 262.

Type material.

Syntype NHMUK 20130214 (4 shells; Fig. 23A ) from Camboja [Cambodia]

Other material examined.

Laos: Tham Pha Tok, Nong Khiaw, Luang Phrabang: CUMZ 10010/1, 10010/2 (Fig. 23E ). Ban Oudom, Meuang Xai, Oudomxay: CUMZ 10006 (Fig. 23D ). Ban Homsai, Meuang Ngeun, Sayaboury: CUMZ 10007, 10008. Ban Na Mone, Sayaboury: CUMZ 10009/1, 10009/2 (Fig. 23B, C ). Thailand: Sa-pan Waterfall, Bo Kluea, Nan: CUMZ 4868 (Fig. 23F ).

Diagnosis.

Shell medium, thin, and depressed. Detached whorl longer than aperture width and descending. Breathing device tubular, and often attached to preceding whorl. Shell with brownish zigzag patterns; thick corneous periostracum.

Differential diagnosis.

Based on the type and museum specimens, R. simplicilabre is very closely allied to R. housei . The differences are the tubular-shaped breathing device perpendicular to the detached whorl, depressed spire, and thickened periostracum. It differs from R. hainesi by having a brownish zigzag colour pattern, shorter detached whorl, and smaller shell size. In comparison, R. hainesi has a thick brownish to dark brown periostracum, longer detached whorl, and larger shell size.

Description.

Shell. Shell medium, cW 20.9-26.1 mm, cH 9.7-12.8 mm, thin, and sub-discoidal to discoidal shape; detached-whorl length 7.5-12.5 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 and regular growth lines. Periostracum thick corneous, and transparent. Shell with brownish zigzag patterns; ventral shell surface with paler colour and less brownish pattern; with narrow dark brown spiral band on periphery. Detached whorl long and almost same length as apertural width, curved and descending. Peristome circular and double; lip somewhat thin and slightly expanded. Aperture opened sub-laterally. Breathing device tubular and its tip usually attached to preceding whorl; outer lip forming short, curved, 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. 23 ).

Distribution.

None of the R. simplicilabre specimens were collected from modern-day Cambodia; therefore, the distribution ranges of this species still need to be verified with specimens from accurate localities within Cambodia (Fig. 18 ).

In this study, R. simplicilabre has been determined to be mainly distributed in the northern part of Laos in Luang Phrabang and Xayabuli provinces, and in northern Thailand. In addition, the specimens recorded from Son La, Dien Bien, and Hao Binh provinces in northern Vietnam have been attributed to R. housei .

Remarks.

This species was described in the same publication by L. Pfeiffer (1862a), immediately after R. hainesi based on specimens in the collection of H. Cuming obtained from Henry Mouhot. The collection locality was brief “Camboja”; it is known that Mouhot’s recorded localities were imprecise and referred to broad geographical areas, for example, “Siam”, "Lao Mountains, Camboja" and “Camboja” (see Páll-Gergely et al. 2016a; Inkhavilay et al. 2019; Pholyotha et al. 2021). This has made it challenging to infer more precise type localities of several land snail species described from Mouhot’s specimens. According to the recorded itinerary of H. Mouhot ( Mouhot 1864: 57; Ashburton 1864: map), land snail surveys from southern Cambodia could not find any specimens matching this species. The most recent works on land snails from southern Cambodia suggest the non-existence of R. simplicilabre (see Sutcharit et al. 2020). We have surveyed several limestone hills near Luang Phrabang areas, Laos, and encountered numbers of empty shells and living specimens that are well-matched with the type specimens of R. simplicilabre in all aspects (Fig. 23B-E ). In addition, the specimens referred to as " Rhiostoma marioni (Ancey, 1898)" in Inkhavilay et al. (2019: fig. 11b) should be recognised as this species.

Figures

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

Figure 23. Shell of Rhiostoma simplicilabre A syntype NHMUK 20130214 from Camboja B, C specimens CUMZ 10009 / 2 from Ngeun, Sayaboury, Laos D specimen CUMZ 10006 from Meuang Xai, Oudomxay, Laos E specimen CUMZ 10010 / 2 from Nong Khiaw, Luang Prabang, Laos F specimen CUMZ 4868 Bor Kluea, Nan.