Chamalycaeus irmatallus Páll-Gergely, 2021

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 : 14

Chamalycaeus irmatallus Páll-Gergely n. sp.

Figs 9 , 10A

Material examined. Holotype: Thailand, Surat Thani Province, limestone mtn., 6 km S of Na San , 8°39.583’N, 99°23.883’E, 100 m a.s.l., 3 Jun. 1987, F.G. Thompson leg., UF 279508 (D: 2.02 mm, H: 1.32 mm) . Paratypes: 11 shells, same data as holotype, UF 551216 . Other material: 6 shells, Myanmar, Thayet-myo, Pegu, coll. Blanford, NHMUK 1906.4.4.71; 1 shell, Thailand, Phang Nga Province, Phang Nga, Tham Nam Phud , 8°27.829’N, 98°32.617’E, 40 m a.s.l. (locality code: 2015/33), 20 Feb. 2015, A. Hunyadi leg., around the cave, HA .

Etymology. The specific epithet is an anagram for armillatus , a species with which this new species has been confused (see Differential diagnosis).

Diagnosis. A tiny Chamalycaeus species with glossy shell, irregularly, finely ribbed R1, very short R2 with blunt, low ribs, and somewhat longer R3 (the two latter regions are shorter than a quarter whorl combined).

Description. Shell off-white, rather glossy, although available shells are somewhat eroded; shell outline slightly oval in dorsal view; spire slightly elevated, conical-domed; body whorl somewhat “parabolic” in apertural view; protoconch rather low, glossy, spiral striae lacking, 1.25 whorls; R1 of 1.5–1.75 whorls, with very indistinct, irregular, low and rounded, relatively wide ribs, and some weak spiral striation; boundary between R1 and R2 distinct due to change in intensity and strength of ribbing; R2 short, only ca. 16–20 ribs; R2+R3 less than quarter whorl (ca. 70–80˚); R3 up to twice as long as R2; boundary between R2 and R3 clear due to change in rib morphology, although constriction very shallow; R3 with very fine, rounded ribs, and low, convex, elongated central swelling; aperture round, slightly oblique to shell axis; boundary between inner and outer peristomes distinct, inner peristome strongly protruding, thickened, slightly expanded; outer peristome slightly weaker or as strong, also expanded, only slightly reflected toward umbilicus; umbilicus relatively narrow, ca. one third of shell width.

Measurements. D: 1.75–2.02 mm, H: 1.15–1.32 mm.

Operculum. Unknown.

Differential diagnosis. Chamalycaeus armillatus ( Benson, 1856) (material examined: Thayet-Mio, UMZC 102995, holotype, Fig. 10B ) has a more rounded body whorl, longer R2 and R3, and a thicker peristome with greater distance between the edges of the inner and outer peristomes.

Distribution. The type locality is in Thailand’s Surat Thani Province. A single shell was collected in the neighbouring Phang Nga Province (locality code: 2015/33, coll. HA). Shells collected more than 1,000 km north in Thayet, Myanmar are so similar to the Thai populations that they could not be separated (see below) ( Fig. 3 ).

Remarks. Chamalycaeus armillatus was described from an eroded specimen. Some spiral striations are visible on the holotype, but they might be the part of the lower shell layer. Thus, this species was provisionally assigned to Chamalycaeus in Páll-Gergely et al. (2020).

Specimens from Thayet (NHMUK 1906.4.4.71, labelled as armillatus ) have slightly less densely arranged R2 ribs than typical C. irmatallus n. sp.

A single, eroded shell from Tham Nam Phud ( Thailand) is somewhat larger than the holotype of C. irmatallus n. sp., and has a longer R2. Since all other shell characters are identical, we provisionally identify the Tham Nam Phud and Thayet specimens as C. cf. irmatallus n. sp.

Figures

FIGURE 3. Distribution of alycaeid species in southern Thailand. Filled triangle: Dicharax ovatus Páll-Gergely & Auffenberg, n. sp., inverted filled triangle: Dicharax subroseus Páll-Gergely, n. sp., inverted empty triangles: Chamalycaeus irmatallus Páll-Gergely, n. sp., empty circles: C. krabiensis Páll-Gergely & A. Reischütz, n. sp., filled circle: Chamalycaeus serratus Páll-Gergely & Auffenberg, n. sp., empty triangles: Chamalycaeus hirpex Páll-Gergely & A. Reischütz, n. sp., filled square: Chamalycaeus buaboke Páll-Gergely & A. Reischütz, n. sp.

FIGURE 9. Chamalycaeus irmatallus Páll-Gergely, n. sp. A: holotype (UF 279508); B: paratype (UF 551216).

FIGURE 10. Shells of Chamalycaeus Möllendorff, 1897 species. A: Chamalycaeus irmatallus Páll-Gergely, n. sp., NHMUK 1906.4.4.71; B: Chamalycaeus (?) armillatus (Benson, 1856), holotype (UMZC 102995).