Dicharax (?) gnomus Páll-Gergely 1900

Páll-Gergely, Barna, Hunyadi, András, Grego, Jozef, Reischütz, Alexander & Auffenberg, Kurt, 2021, Nineteen new species of Alycaeidae from Myanmar and Thailand (Gastropoda: Caenogastropoda: Cyclophoroidea), Zootaxa 4973 (1), pp. 1-61 : 29-31

Dicharax (?) gnomus Páll-Gergely n. sp.

Fig. 22

Material examined. Holotype: Thailand, Phrae Province, 4 km N of Ban Nim Rin , valley with limestone ridges, 18°10.133’N, 99°56.233’E, 200 m a.s.l., 16 May 1988, F.G. Thompson leg., leaf litter at base of boulders, UF 380401 (D: 1.65 mm, H: 1.15 mm).

Etymology. The specific epithet gnomus (Medieval Latin for gnome) refers to the tiny shell that has unusual shell area ratios compared to most other species (to be used as a noun in apposition).

Diagnosis. Tiny Dicharax species with wide, flat R1 ribs and fine spiral striation, short R2 with flat ribs, strongly reduced R3, and circular aperture.

Description. Shell light grey, slightly semi-translucent; shell outline rounded in dorsal view; spire slightly elevated, body whorl rounded; protoconch low, finely granular, glossy, 1.5 whorls; R1 of 1.25 whorls, beginning with low ribs that appear curved anteriorly, developing into wide, flattened ribs separated by grooves, forming a smoothish, glossy surface; first whorl of R1 with some fine spiral striation; boundary between R1 and R2 not conspicuous due to similar rib morphology, although ribs become denser, ca. 30 R2 ribs present; R2+R3 less than quarter whorl (ca. 70–80˚); R3 considerably shorter than R2; boundary between R2 and R3 distinct due to sculpture change and rather deep constriction; R3 strongly reduced, represented only as a slight swelling just behind peristome; aperture almost parallel with shell axis, circular, round; peristome weak, very slightly thickened; boundary between inner and outer peristomes barely discernible; inner peristome weak, somewhat protruding and very slightly expanded, outer peristome very weakly developed, only indicated; umbilicus relatively narrow, less than one third of shell width.

Measurements. D: 1.65 mm, H: 1.15 mm.

Operculum. Unknown.

Differential diagnosis. The R3 region practically absent in this species. No other low-spired alycaeid species has this trait in Thailand or Myanmar. The Dicharax species described as Awalycaeus Kuroda, 1951 in Japan also have a strongly reduced R3, but we consider this is a case of convergent evolution.

Distribution. This new species is known only from the type locality ( Fig. 17 ).

Remarks. This species has unusual conchological characters, and therefore, its generic placement is unclear. The spiral striation on R1 and the lack of spiral striation on the protoconch suggests a relationship to Chamalycaeus , whereas the glossy shell surface, the flattened R2 ribs, and the low protoconch indicate a position in Dicharax . This study revealed other Dicharax species /populations that possess some spiral striae on R1 (see under Dicharax admirandus , n. sp. and D. omissus and Páll-Gergely et al. 2017). Thus, the placement of this species in Dicharax is justified.

Figures

FIGURE 17. Distribution of alycaeid species in northern Thailand and Myanmar. filled circle: Dicharax admirandus Páll- Gergely & Auffenberg, n. sp., empty square: Dicharax ebrius Páll-Gergely & Auffenberg, n. sp., filled triangle: Dicharax gnomus Páll-Gergely, n. sp., filled square: Dicharax mirounga Páll-Gergely, n. sp., inverted empty triangles: Dicharax verruco- sus Páll-Gergely & Hunyadi n. sp., rhombus: Dicharax stuparum Páll-Gergely & Hunyadi, 2018, empty triangle: Metalycaeus anas Páll-Gergely & A. Reischütz, n. sp.

FIGURE 22. Dicharax (?) gnomus Páll-Gergely, n. sp., holotype (UF 380401).