Rhiostoma gnomus Tongkerd & Panha, 2023

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

26. Rhiostoma gnomus Tongkerd & Panha sp. nov.

Figs 44 , 51 , 53C

Type material.

Holotype CUMZ 4716 (W 16.4 mm, H 10.7; Fig. 51A ). Paratypes CUMZ 3922 (55 adults + 31 juveniles), CUMZ 4717 (3 shells; Figs 51B-D , 53C ), CUMZ 4718 (3 shells; Fig. 51F ), CUMZ 4849 (9 shells; Fig. 51E ), CUMZ 4872 (16 adults + 2 juveniles), NHMUK 20220447 (5 shells), and SMF 368682 (5 shells). All paratypes are from the type locality.

Type locality.

Limestone hill with dry deciduous forest at Pha Chu (Srinan), Na Noi District, Nan Province, Thailand (18°22'4.6"N, 100°50'23.4"E).

Other material examined.

Thailand: Sri Nan, Na Noi, Nan: CUMZ 10035 .

Diagnosis.

Shell small and heliciform; detached whorl shorter than apertural width. Shell colour usually with brownish zigzag patterns. Breathing device notch-shaped. Operculum calcareous, low cup-shaped.

Differential diagnosis.

This new species differs from all other members in this species group by having a small and heliciform shell, sometimes with a very short detached whorl and notch-shaped breathing device. Except for R. cambodjense , all other species have a flattened or sub-discoidal shell and a canal-shaped breathing device. Rhiostoma gnomus sp. nov. differs from R. cambodjense by having a relatively smaller shell size, lighter shell colour, and apex without dark colour. In contrast, R. cambodjense tends to have a larger shell and darker shell colour.

Description.

Shell. Shell small, width 14.6-18.2 mm, height 8.9-11.1 mm, thin, and heliciform. Apex acute; spire elevated. Whorls 4 to 5, convex, increasing regularly; suture wide and deep; last whorl rounded. Shell surface smooth or with fine growth lines. Shell colour usually with brownish zigzag patterns or uniformly white to brown; narrow and brownish spiral band usually present (or absent in albinistic forms). Periostracum thin, corneous, and brownish colour. Detached whorl very short and slightly descending. Peristome circular and double; lip slightly thickened and rarely expanded. Aperture opened sub-laterally. Breathing device notch-shaped; outer lip protruding, with shallow groove; inner lip with shallow incision. Umbilicus widely opened and deep. Operculum calcareous, low cup-shaped, and multispiral (Fig. 51 ).

Radula. Teeth arrangement and shape are similar to those of R. asiphon . Central tooth with large central cusp and two lateral cusps on each side. Lateral teeth composed of four cusps; central cusp triangular; two small inner cusps with pointed tips and tapering in size, and one small outer cusp. Marginal teeth each composed of three cusps (Fig. 53C ).

Etymology.

The species name is derived from the Latin word gnomus, meaning dwarf. It refers to the small shell size of the new species.

Distribution.

This new species is known only from the type locality at the base of a limestone cliff in Nan Province, Thailand, near the Thai-Laos border (Fig. 44 ).

Remarks.

Although this species is superficially similar to the genus Cyclotus , its calcareous, cup-shaped and multispiral operculum with elevated lamellae and its incomplete tube as breathing device on the apertural lip suggests it is a member of genus Rhiostoma . Shell colour variation is observed in the type population varied from a brownish zigzag pattern, pale brown blotches to monochrome whitish, or the albinistic shell form found sympatrically within the type population and here recognised as colour variation.

Figures

Figure 44. Distribution map of Rhiostoma morleti - species group.

Figure 51. Shell of Rhiostoma gnomus sp. nov. A holotype CUMZ 4716 from Pha Chu, Na Noi, Nan B-D paratypes CUMZ 4717 and E paratype CUMZ 4849 from type locality F paratype CUMZ 4718 from type locality shows albinistic shell form.

Figure 53. Radula morphology A Rhiostoma cambodjense, specimens CUMZ 4714 from Khao Chakan, Srakeo B Rhiostoma cheliopegma sp. nov., paratype CUMZ CUMZ 3985 / 2 Tham Takien, Khao Chamao, Rayong C Rhiostoma gnomus sp. nov. paratype CUMZ 4717 from Pha Chu, Na Noi, Nan D Rhiostoma laosense sp. nov. specimen CUMZ 10205 from Tad Muang, Khammouane, Laos.