Dicharax cyclostoma Pall-Gergely 2021, n. sp.

Dicharax cyclostoma Páll-Gergely n. sp.

Fig. 19

Material examined. Holotype: Thailand, Nakhon Ratchasima Province, 3.4 km W of Ban Mu Si, limestone hill, 14°32.016’N, 101°22.783’E, 380 m a.s.l., 5 May 1987, F.G. Thompson leg., UF 346812 (D: 2.54 mm, H: 1.54 mm). Paratypes: 18 shells, same data as holotype, UF 551219. Other material: 2 juvenile shells, same data as holotype, UF 551220.

Etymology. This species is named for the very round aperture (Greek cyclo-, ring and stoma, mouth, opening, noun in apposition).

Diagnosis. A small, depressed Dicharax species with glossy R1, protruding inner peristome, and round aperture; R2+R3 shorter than a quarter whorl, R2 half as long as R3.

Description. Shell white, some specimens with very faint brownish or yellowish colouration; shell outline rounded or very slightly oval in dorsal view, shell depressed, spire only slightly elevated, body whorl rounded; protoconch low, glossy, 1.25–1.5 whorls; R1 of 1.25–1.75 whorls, glossy without sculptural elements; boundary between R1 and R2 conspicuous due to R2 ribs; R2 with ribs curved towards aperture, ca. 24–26 ribs present; spaces between R2 ribs narrower than ribs; R2+R3 less than quarter whorl (ca. 80˚); R2 half or less length of R3; R3 glossy, smooth; boundary between R2 and R3 distinct due to change in sculpture and moderately deep constriction; middle of R3 with rounded swelling; aperture strongly oblique to shell axis, round; boundary between inner and outer peristomes not conspicuous; inner peristome somewhat protruding, rather slim, outer peristome multi-layered, thickened and expanded, particularly toward umbilicus, but not reflected; umbilicus wide, slightly more than a third of shell width.

Measurements. D: 2.22–2.68 mm, H: 1.31–1.56 mm.

Operculum. Unknown.

Differential diagnosis. Dicharax politus (W.T. Blanford, 1865) (examined material: Phungdo, Arakan, coll. Blanford, NHMUK 1906.4.4.178, 3 probable syntypes, Fig. 20C) is larger, has a more elevated spire, and longer R3. Dicharax (?) immaculatus Páll-Gergely, 2017 from Gansu Province, China is similar with low spire and smooth R1, but with shorter R2 without elevated ribs, and its peristome much thinner than that of new species. Some populations of D. cristatus (Möllendorff, 1886) possess a smooth R1 and a low spire, but it has a longer R2, and is always larger than the new species (Páll-Gergely et al. 2017).

Distribution. This new species is known only from the type locality (Fig. 5).