Alycaues roebeleni Moellendorff , 1894

Foon, Junn Kitt & Liew, Thor-Seng, 2017, A review of the land snail genus Alycaeus (Gastropoda, Alycaeidae) in Peninsular Malaysia, ZooKeys 692, pp. 1-81 : 40

Alycaues roebeleni Moellendorff, 1894 Figures 26, 31W

Alycaeus roebeleni Möllendorff, 1894: 146-156; Tarruella and Domènech 2011: 73, figure 3A.

Alycaeus (Alycaeus) roebelini [sic]: Laidlaw 1928: 35.

Alycaeus (Alycaeus) roebeleni : Zilch 1957: 147, plate 6, figure 27.

Type locality.

Samui Islands (= Koh Samui archipelago), Thailand.

Type material.

Lectotype . Koh Samui, Thailand: SMF 109317 (Seen).

Other examined materials.

PRS 11 Bukit Tok Seri, Perlis: BOR/MOL 6863/1, BOR/MOL 8368/2. PRS 17 Gunung Chabang, Perlis: BOR/MOL 6230/1, BOR/MOL 6869/1, BOR/MOL 6873/6, BOR/MOL 8374/8. PRS 19 Bukit Chuping, Perlis: BOR/MOL 6859/1, BOR/MOL 8363/30, BOR/MOL 12980/1, ZRC 1975.2.22.99-126/29. Koh Samui, Thailand: RMNH 153575/2, ZMA 317517/1, ANSP 81380/2. Mountain north of Phatthalung, Thailand: ANSP 446393/1.

Description.

Protoconch. Smooth.

Shell shape. Conical. Shell height: 7.20-8.01 mm. Shell width: 8.94-10.08 mm.

Spire. Spire height: 2.17-2.74 mm. Spire width: 3.13-3.68 mm. Number of whorls: up to 4 ½. Spire shape: oblong conical. Whorl periphery rounded. Umbilicus open.

Whorl constriction. At about 4 ¼ whorls posterior of protoconch.

Breathing tube. Length: 0.71-0.94 mm.

Aperture and peristome. Aperture circular, very expanded. Aperture height: 4.39-5.21 mm. Aperture width: 4.08-5.01 mm. Peristome single to double, not thickened. Peristome orientation 44-49° oblique with respect to the coiling axis.

Spiral lines. Indistinct. Regularly spaced. Approximately 23-37 lines per 1 mm.

Radial ribs running anterior of breathing tube. Radial ribs pronounced, evenly spaced. Approximately 8-9 ribs per 1 mm.

Radial ribs running perpendicular to breathing tube. Radial ribs pronounced, evenly spaced. Approximately 7-10 ribs per 1 mm.

Radial ribs running posterior of breathing tube. Radial ribs pronounced, unevenly spaced immediately posterior of constriction.

Operculum. Concave, rounded. Exterior covered with thin calcareous layer, smooth. Interior covered with thick proteinaceous coating, smooth, mamillated.

Shell colour. Yellow at apical whorls. Either fades to white towards ultimate whorl or remain yellow throughout.

Living animal. Unknown.

Habitat and ecology.

Lives on limestone rock crevices covered by mosses. In shady forests on limestone hills.

Distribution range.

Restricted to limestone hills of the Chuping geological formation in central Perlis. Elsewhere, in Koh Samui and Phatthalung, southern Thailand ( Möllendorff 1894, materials examined).

Differential diagnosis.

Alycaeus roebeleni is most similar to A. perakensis in shell shape but is larger (larger by about 0.97 mm in shell height, 1.96 mm in shell width), spire taller and wider (taller by about 0.22 mm in spire height, wider by about 0.31 mm in spire width), penultimate whorl wider as well as peristome more expanded ( Möllendorff 1894). Alycaeus roebeleni also differs from A. perakensis in its operculum, which has a smooth exterior and thick, brown proteinaceous layer at the interior. The spire height of A. roebeleni is quite variable across its range but never as tall as A. perakensis .

Discussion.

This species was considered closely related to A. perakensis by Möllendorff (1894), which we concur with, based on examined specimens. However, contrary to the Möllendorff (1894) claim that the operculum is entirely proteinaceous, we show that A. roebeleni has proteinaceous interior and calcareous exterior layers. Despite clear distinctions made between A. roebeleni and A. perakensis in Möllendorff (1894), the two species remain confused by past workers. An A. roebeleni collection lot in ZRC was originally labelled as A. perakensis (ZRC 1975.2.22.99-126). A specimen labelled as A. roebeleni from Banang Pupo, Yala, Thailand is actually A. perakensis (ANSP 446394). Alycaeus roebeleni minor Möllendorff, 1894, from Biserat, Yala, Thailand is also likely A. perakensis (see Remarks in A. perakensis ). The Perlis population appears to be the southern-end of the distribution range for A. roebeleni .