Rhiostoma erawanense Prasankok & Pimvichai 2026, sp. nov.

Rhiostoma erawanense Prasankok & Pimvichai sp. nov. Figs 5, 7 C, D Rhiostoma dalyi Tongkerd et al. 2023: fig. 25 (in part). Material examined. Holotype Thailand • 1 shell (height 10.5 mm, width 21.5 mm; Fig. 5 A); Phrae Province, Long District, Tham Erawan; 18°01'59.8"N, 99°30'21.0"E; P. Pimvichai leg.; CUMZ 15480. Paratypes Thailand • 20 shells; same collection as for Holotype; CUMZ 15480 (Fig. 5 B – F). Other material. Thailand • 10 shells; same data as for holotype; CUMZ 15480 /1. Diagnosis. The shell is medium-sized and covered with a thin, pale brown periostracum. The detached whorl is distinctly separated from the penultimate whorl, curved, descending, and equal to the aperture width (mean detached whorl length / aperture width ratio = 1.00). The breathing device is small, tubular, of medium length, and slightly curved. Differential diagnosis. Rhiostoma erawanense sp. nov. is geographically and phylogenetically close to R. dalyi but differs by having a smaller shell, a medium-length, small tubular breathing device, and a detached whorl length equal to the aperture width. In contrast, R. dalyi has a larger shell, a longer, curved tubular breathing device, and a mean detached-whorl length-to-aperture width ratio of 1.07. Description. Shell. Shell medium-sized (height 9.0–11.0 mm, width 19.0–21.0 mm, detached-whorl length 4.5–10.0 mm), thick, discoidal, widely umbilicate. Apex acute with dark color; spire little elevated. Whorls 4–5 convex, increasing regularly; suture wide and depressed; last whorl rounded; detached whorl medium-length, curved and descending. Shell surface with thin growth lines. Shell color pale brownish or with thin zigzag streaks, accompanied by a thin, or sometimes thick, dark brown peripheral band. Periostracum thin corneous, brownish. Aperture rounded; lip thickened, two layers; inner lip, outer lip with slightly expanded. Breathing device small, tubular, medium-length and slightly curved; small hole. Operculum calcareous, low cup-shaped and multispiral. Radula. Taenioglossan radula, each row contains seven teeth with formula 2–1 – 1 – 1 – 2 (Fig. 7 C). Central tooth large and semicircular with five cusps, large and flattened at center and smaller cascades laterally. Lateral teeth with four pointed cusps; two inner cusps small but larger than outermost, third cusp triangular and largest, one outer cusp smallest. Marginal teeth tricuspid, two layers; inner marginal teeth located at a slightly lower position than central tooth, center large and triangular, smaller lateral cusps, the outer marginal teeth located at the edge of the ribbon with a slightly lower position than lateral teeth, located at a slightly higher position than central tooth with central cusp smaller than on inner marginal teeth. Jaw. Jaw sculpture with very strong longitudinal ridges, connected to each other with disordered or ordered parallel slanting transverse ridges, resulting in approximately regular pattern of parallel adjacent lanes of slightly bent oblique rectangles (Fig. 7 D). The general structure of the jaw is consistent with the typical morphology observed across the genus Rhiostoma. Etymology. The name refers to Tham Erawan, the type locality of this species. Distribution. Known only from the type locality in Phrae Province, Thailand. Specimens were collected under leaf litter near a river. Remarks. Rhiostoma erawanense sp. nov. was previously classified as R. dalyi (Tongkerd et al. 2023); however, it is supported as clade B in the phylogenetic tree (Fig. 2) and in the morphological analysis (Table 3).