Metalycaeus aries Pall-Gergely & Auffenberg 2021, n. sp.

Metalycaeus aries Páll-Gergely & Auffenberg, n. sp.

Figs 38, 39A, 40A–D

Material examined. Holotype: Thailand, Loei Province, limestone knoll 1 km E Ban Huai Muang, top of knoll, 17°28.433’N, 101°53.333’E, 375 m a.s.l., 21 May 1987, F.G. Thompson leg., in leaf litter, UF 343786 (D: 2.48 mm, H: 1.49 mm). Paratypes: 12 shells, same data as holotype, UF 551225. Other material: 1 shell (Fig. 38B, D: 2.2 mm, H: 1.61 mm) + 5 shells + 4 juvenile shells, Thailand, Nakhon Sawan Province, limestone hill 4.0 km NW of Man Non San, 15°56.517’N, 99°52.400’E, 100 m a.s.l., 12 Jun. 1987, F.G. Thompson leg., UF 279699; 1 shell (Fig. 38C, D: 2.04 mm, H: 1.25 mm) + 3 shells, Thailand, Phetchabun Province, 24.4 km S Phetchabun on Highway 2275 evergreen forest in back of outcrop, base of cliff, 16°16’N, 101°10’E, 120 m a.s.l., 02 May 1988, K. Auffenberg leg., UF 346105.

Etymology. The shape of the shell of this new species is reminiscent of a coiled ram’s horn (aries in Latin). The specific epithet is used as a noun in apposition.

Diagnosis. A tiny Metalycaeus species with strongly ribbed R1, short combined R2+R3, dense, curved R2 ribs, and an angled R2 visible from dorsal side.

Description of the type series. Shell off-white, light grey, although all available shells dead collected; shell outline slightly oval in dorsal view, R2 conspicuously swollen/angled; spire somewhat elevated, low conical; body whorl rounded with distinct lower, keel caused by angled ribs; protoconch of 1.5 whorls, elevated, first ca. 0.75 whorl finely granular and glossy, the last 0.75 whorl spirally striated; R1 of 1.5–1.75 whorls, with strong, equidistant, narrow ribs and fine, dense spiral striae between ribs; boundary between R1 and R2 distinct due to densely-placed R2 ribs; R2+R3 less than a quarter whorl (ca. 70˚); R2 swollen, forming blunt angle in dorsal view, and rounded in apertural view; R2 ribs curved toward aperture, nearly adnate, ca. 24–26 R2 ribs present; R2 and R3 of comparable length; R3 with spiral striation and weaker ribs than R1; boundary between R2 and R3 distinct due to lower R3 ribs and descending portion of R2 angle; aperture slightly oblique to shell axis, round; boundary between inner and outer peristomes distinct; inner peristome protruding and expanded, relatively thin; outer peristome thicker, wider, expanded, and very slightly reflected toward umbilicus; umbilicus moderately narrow, approximately one third of shell width.

Measurements. 2.04–2.54 mm, H: 1.3–1.61 mm.

Operculum. An operculum was present in one specimen (UF 279699). Outer surface multispiral without elevated lamina, originally covered with mud (a portion left on operculum for photography; Fig. 38B), inner surface with very small, low, central nipple.

Differential diagnosis. The only similar species is M. bicarinatus n. sp., which has two distinct keels in R2, has slimmer, more elevated, and more widely-spaced ribs, and the protoconch lacks obvious spiral striae.

Distribution. Three populations are known from the Thai provinces of Loei, Nakhon Sawan, and Phetchabun (Fig. 5).

Remarks. The three populations classified as M. aries n. sp. show some conchological variability, which is compiled in Table 4.