Description for Peninsular Malaysian species. Protoconch. Smooth. In some species, second whorl grooved. Shell shape and size. Conical (pyramidal), flat or globose. Shell height: 3.03- 10.91 mm. Shell width: 3.36-15.04 mm. Spire. Spire height: 0.93-3.21 mm. Spire width: 1.40-4.00 mm. Number of whorls: up to 3-5 1/2. Spire shape always oblong conical. Whorl periphery strongly keeled or rounded. Always umbilicated. Umbilicus open or partially closed. Whorl constriction. Position of constriction about 2 1/4-5 1/4 whorls posterior of protoconch. Breathing tube. Always present, varies in length: 0.29-6.52 mm. Aperture and peristome. Aperture always circular. Apertural margin moderately expanded or very expanded. Aperture height: 1.59-7.30 mm. Aperture width: 1.60- 6.59 mm. Peristome single, double or triple. Peristome thickened or not thickened. Peristome notched or winged at suture. Interspace between peristome none, narrow or wide. Peristome orientation varies 4°-55° oblique to the coiling axis. Spiral lines. Absent or present. When present, distinction varies (distinct or indistinct), spacing always regular to irregular. Number of spiral lines: 11-60 lines per 1 mm. Radial ribs running anterior of breathing tube. Pronounced, indistinct or absent. Even or unevenly spaced. 6-21 radial ribs per 1 mm. Radial ribs running perpendicular to breathing tube. Pronounced, indistinct or absent. Radial ribs sometimes are thicker and whiter compared to radial ribs anterior of breathing tube. Even or unevenly spaced. 6–29 ribs per 1 mm. Radial ribs running posterior of breathing tube. Pronounced, indistinct or ab- sent. Usually unevenly spaced. 2-25 ribs per 1 mm. Operculum. Concave or flat. Rounded or conical. Exterior usually calcareous layered of variable thickness. Exterior texture varies (smooth, finely granulated, flaky,short calcareous spikes, cup-like projection at nucleus, scaffold-like calcareous deposits or appressed radially spiral lamellae). Interior usually proteinaceous layered. Interior sculpture smooth, with multilamellar impression or mamillated. Shell colour. Varies between and within species (white, pinkish-white, pink, yellow, orange, red, brownish-red. brown, purple). Apical whorls either in white or non- white colours, always fading to white in subsequent whorls. Living animal. Body colour variable but lighter than head and tentacle (cream- white, yellow, cream-yellow, light brown, brown, maroon, greenish-black, light grey, grey). Head colour variable (orange, pink, pinkish-brown, brown, maroon, greenish- black, grey, dark grey). Tentacle either uni-coloured (yellow, red, brown, reddish- brown, light grey, dark grey, black), or bi-coloured (green with brown tips, cream- white with pink tips, yellow with red tips). Head and tentacle often darker coloured relative to the body. Habitat and ecology. Three distinct habitats: rock (crevices, rock walls, solution holes, moss and lichen covered rocks), vegetation (low shrubs, tree trunks) or forest floor (rotten logs, leaf litter). Usually in moist areas but ocassionally found in drier areas. Always in forested habitats. Remarks. The genus Alycaeus Baird, 1850, is separated into three subgenera - Alycaeus Baird, 1850, Pincerna Preston, 1907, and Stomacosmethis Bollinger, 1918 (Egorov 2013). However, all of these subgenera (including Pincerna (see Páll-Gergely 2017)) remain inadequately defined as no clear diagnostic characters have ever been identified (Godwin-Austen 1889) and hence will require extensive revision, a task that is beyond the scope of this study and will instead be treated in later studies (B. Páll-Gergely, pers. comm.). Thus, we do not provide a diagnosis for the genus Alycaeus here pending a revision of the Alycaeidae and resolution of diagnostic characters useful for supraspecific groups. Instead, consistent with our study scope, we will only provide a description of characters relevant to Peninsular Malaysian Alycaeus species.