Rhiostoma lampangense Prasankok & Pimvichai 2026, sp. nov.

Rhiostoma lampangense Prasankok & Pimvichai sp. nov. Figs 6, 7E, F Material examined. Holotype Thailand • 1 shell; (height 11.8 mm, width 20.2 mm; Fig. 6 A); Lampang Province, Mae Tha District, Tham Phra Sabai; 18°05'34.0"N, 99°32'13.0"E; P. Pimvichai leg.; CUMZ 15481. Paratypes Thailand • 20 shells; same collection as for holotype; CUMZ 15481 (Fig. 6 B, C). Other material. Thailand • 10 Shells; same data as for holotype; CUMZ 15481 /1 • 15 shells; Lampang Province, Maung Pan District, Ban Khao Subdistrict, Tham Itthirit; 18°36'12.3"N, 99°29'37.4"E; CUMZ 15482 (Fig. 6 D, E) • 15 shells; Lampang Province, Maung Pan District, Ban Khao Subdistrict, Doi Phrabath; 18°36'46.4"N, 99°29'51.9"E; CUMZ 15483 (Fig. 6 F). Diagnosis. The shell is small to medium-sized and heliciform, covered with a thick, dark brown periostracum. A detached whorl is completely absent. The breathing device forms a distinct canal that is triangular in shape. Differential diagnosis. Rhiostoma lampangense sp. nov. is geographically close to R. panhai but differs in lacking a detached whorl, while R. panhai has a medium-length and slightly descending detached whorl (mean detached-whorl length: aperture width ratio = 0.86). Description. Shell. Shell small to medium-sized (height 14.1–17.5 mm, width 23.4–27.0 mm), thick, heliciform, widely umbilicate. Apex acute; spire elevated. Whorl 4–5 convex, increasing regularly; suture wide and depressed; last whorl rounded, without detached whorl. Shell surface with thin growth lines. Shell color: highly variable among populations. Type locality: dark brownish with a dark brown peripheral band (usually thick to double-thick, but sometimes thin). Tham Itthirit: pale brownish or with zigzag streaks, accompanied by a dark brown peripheral band (thin or sometimes thick). Doi Phrabath: pale brown with thin zigzag streaks, and a dark brown peripheral band (thin, thick, or double-thick). Aperture rounded; lip thickened, two layers; inner lip; with or without shallow incision, outer lip; breathing device short, wide, triangular canal located at base near suture. Operculum calcareous, low cup-shaped, and anticlockwise multispiral. Radula. Taenioglossan radula, each row contains seven teeth with formula 2–1 – 1 – 1 – 2 (Fig. 7 D). 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 l 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 same level as central tooth with central cusp smaller than on inner marginal teeth. Jaw. Jaw sculpture with very strong longitudinal ridges, connected to each other with regular parallel slanting transverse ridges, resulting in approximately regular pattern of parallel adjacent lanes of slightly bent oblique rectangles (Fig. 7 F). The general structure of the jaw is similar to that of R. erawanense sp. nov. and R. phuphamanense sp. nov. but differs from R. lampangense sp. nov. by having shorter, parallel, slanting transverse ridges. Etymology. The name refers to Lampang Province, where the type locality of this species is located. Distribution. Lampang Province, Thailand. Specimens were collected under leaf litter near Limestone Mountain. Remarks. Rhiostoma lampangense sp. nov. clusters with R. panhai and R. phunangense but is distinguished by its complete lack of a detached whorl, a small- to medium-sized heliciform shell with a dark brown periostracum, and a triangular breathing canal. In contrast, R. panhai has a medium-sized shell with a short-to-medium detached whorl (mean detached whorl length / aperture width ratio = 0.86), a dark brown shell with paler zigzag streaks, and a small, slender, curved tubular breathing device (Prasankok 2023). Similarly, R. phunangense differs from R. lampangense sp. nov. by having a medium-to-large shell with a medium-length detached whorl (mean detached whorl length / aperture width ratio = 0.82), a dark brown shell with paler zigzag streaks, and a larger, curved, medium-length tubular breathing device (Prasankok et al. 2023). This species is supported as clade C in the phylogenetic tree (Fig. 2) and in the shell morphological analysis (Table 3).