Rhiostoma phuphamanense Prasankok & Pimvichai 2026, sp. nov.

Rhiostoma phuphamanense Prasankok & Pimvichai sp. nov. Figs 4, 7 A, B Rhiostoma ebenozostera Tongkerd et al. 2023 (in part). Material examined. Holotype Thailand • 1 shell (height 13.0 mm, width 26.0 mm; Fig. 4 A); Khonkaen Province, Chum Phae District, Tham Poo Loup; 16°49'45.0"N, 101°59'08.0"E; P. Prasankok leg.; CUMZ 15477. Paratypes Thailand • 30 shells; same collection as for holotype; CUMZ 15477 (Fig. 4 B). Other material. Thailand • 15 shells; Loei Province, Wat Khao San Hin; 17°10'40.0"N, 101°57'59.0"E; CUMZ 15479 (Fig. 4 C, D) • 15 shells; Loei Province, Wat Tham Had Nimitre; 17°03'21.0"N, 101°54'48.0"E; CUMZ 15478 (Fig. 4 E, F). Diagnosis. The shell is large, thick, and depressed. The detached whorl is short, slightly descending, and shorter than the aperture width (mean detached whorl length / aperture width ratio = 0.51). The breathing device is short, tubular, and incomplete. Differential diagnosis. Rhiostoma phuphamanense sp. nov. differs from the sister taxon, R. ebenozostera, by having a larger shell and a shorter detached whorl. In contrast, R. ebenozostera features a distinctly longer, descending detached whorl (mean DWL / AW ratio = 1.05). Description. Shell. Shell large (height 11.0–14.0 mm, width 22.5–25.0 mm, detached-whorl length 2.0–5.0 mm), thick, sub-discoidal to discoidal, widely umbilicate. Apex acute; spire little elevated. Whorls 4 to 5 convex, increasing regularly, suture wide and depressed; last whorl rounded; detached whorl very short, slightly descending. Shell surface with thin growth lines. Shell color pale brownish or with thin zigzag streaks, accompanied by a dark brown peripheral band (either thin, thick, or double-thick). Periostracum thick corneous, brownish. Aperture rounded; lip thickened, two layers; inner lip, outer lip, little expanded. Breathing device incomplete tubular, short length, straight; shallow incision. Operculum calcareous, cup-shaped and multispiral. Radula. Taenioglossan radula, each row contains seven teeth with formula 2–1 – 1 – 1 – 2 (Fig. 7 A). 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 same level as 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 lower 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 B). The general structure of the jaw is consistent with the typical morphology observed across the genus Rhiostoma. Etymology. The name comes from the name of the natural park where the type locality of this species is situated. Distribution. Known from the type locality in Khonkaen Province and the nearby area in Loei Province, Thailand. Specimens were collected under leaf litter. They are sympatric with Pollicaria sp. and Cyclophorus sp. Remarks. Rhiostoma phuphamanense sp. nov. was previously classified as R. ebenozostera (Tongkerd et al. 2023; fig. 29); however, R. phuphamanense sp. nov. differs from R. ebenozostera by having a larger shell, shorter detached whorl, and shorter incomplete breathing device. This species is supported as clade A in the phylogenetic tree (Fig. 2) and the shell morphological analysis (Table 3).