Dicharax omissus (Godwin-Austen, 1914)

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 : 38-40

Dicharax omissus (Godwin-Austen, 1914)

Figs 29–30

Alycaeus omissus Godwin-Austen, 1914: 411 , Plate 155, Fig. 13 .

Alycaeus (Chamalycaeus) omissus — Gude 1921: 231.

Material examined. Lectotype (hereby designated): 1 shell, “ Siam & Shan boundary, coll. Woodthorpe ”, NHMUK 1903.7 .1.1228.1 . Paralectotype: 1 shell, same data as for lectotype, NHMUK 1903.7 .1.1228. Other material: 1 shell ( Fig. 29B, D : 2.94 mm, H: 1.94 mm) + 10 shells, Myanmar, Shan State, Hopong centre 5.8 km — Namsang , left side of rd. #4, near „Hopong Spring Cave”, 20°49.028’N, 97°13.469’E, 1110 m a.s.l. (locality code: 2018/35), 06 Oct. 2018, A. Hunyadi, K. Okubo & J.U. Otani leg., HA ; 8 shells, Myanmar, Shan State, Taunggyi centre ca. 16 km — Hopong , rd. #4 N 1.5 km, „Shwe Pyi Aunchonda” monastery, 20°47.263’N, 97°8.239’E, 1110 m a.s.l. (locality code: 2018/46), 08 Oct. 2018, A. Hunyadi, K. Okubo & J.U. Otani leg., HA ; 1 shell ( Fig. 30A, D : 2.81 mm, H: 1.8 mm) + 7 shells, Thailand, Chiang Mai Province, 1.2 km N of Wat Tham Klaeb , 500 m side road from the junction, 19°33.918’N, 99°3.690’E, 650 m a.s.l. (locality code: 2015/08), 08 Feb. 2015, A. Hunyadi leg., HA ; 1 shell ( Fig. 30B, D : 2.69 mm, H: 1.75 mm) + 2 shells, Thailand, Mae Hong Son Province, 800 m from Tham Nam Lod toward the Buddhist temple, 19°33.928’N, 98°16.794’E, 650 m a.s.l. (locality code: 2015/12), 09 Feb. 2015, A. Hunyadi leg., HA .

Type locality. “ Siam and Shan boundary” .

Diagnosis. A variable Dicharax species with a relatively densely, regularly ribbed R1, rarely with spiral striation, dense R2 ribs, a rounded central swelling on R3, and an expanded, but not reflected outer peristome.

Measurements. D: 2.52–2.87 mm, H: 1.71–1.89 mm (2018/08 and 2015/12); D: 2.89–3.23 mm, H: 1.88–2.3 mm (2018/35 and 2015/46).

Differential diagnosis. Typical Dicharax notus and D. omissus are very similar, and since only eroded specimens of the latter species were available, the distinction of these two species was somewhat questionable until newly collected material became available. These specimens reveal that D. notus is generally larger, R1 bears denser ribs (although some populations of D. notus have less dense R2 ribs), and its outer peristome is more reflected.

Distribution. Dicharax omissus is known from the Shan States of Myanmar and neighbouring Thai provinces of Chiang Mai and Mae Hong Son ( Jirapatrasilp et al. 2021; Fig. 28 ).

Remarks. The syntypes and the newly collected 4 samples (possibly 3 populations) show some variability in terms of the sculpture of R1 and R3, and the presence or absence of the notch at the basal portion of the aperture. However, all of these traits are rather minor and do not justify the distinction of species. The conchological differences of these populations are summarized in Table 3 .

The shells of the two samples from Hopong have some slight spiral striation on the R1, which is highly unusual in Dicharax . The spiral striae of this species might not be homologous with those of other alycaeid genera, although this requires further investigation.

Figures

FIGURE 13. Chamalycaeus spiratus Páll-Gergely, n. sp., holotype (UF 279157).

FIGURE 28. Distribution of Dicharax omissus (Godwin-Austen, 1914) (circles) and Dicharax notus (Godwin-Austen, 1914) (triangles). Sample codes correspond with those in the list of examined material and Tables 2 and 3.

FIGURE 29. Dicharax omissus (Godwin-Austen, 1914). A: lectotype (NHMUK 1903.7.1.1228.1); B: site 2018/35.

FIGURE 30. Dicharax omissus (Godwin-Austen, 1914). A: site 2015/08 (coll. HA); B: site 2015/12 (coll. HA).