Georissa borneensis Smith, 1895

Zacaery Khalik, Mohd, P. Hendriks, Kasper, J. Vermeulen, Jaap & Menno Schilthuizen,, 2019, Conchological and molecular analysis of the " non-scaly " Bornean Georissa with descriptions of three new species (Gastropoda, Neritimorpha, Hydrocenidae), ZooKeys 840, pp. 35-86 : 43-44

Georissa borneensis Smith, 1895 Figures 1A, 5 A–K

Georissa borneensis Smith, 1895: 126, plate IV fig. 18; Thompson and Dance 1983: 122, figs 18, 61-62.

Type locality.

Gomanton, N.E. Borneo.

Type material.

Lectotype (Designation by Thompson and Dance 1983) (Fig. 1A). Gomanton, N. Borneo: NHMUK 1894.7.20.61 (glued on paper). Paralectotypes. Gomanton, N. Borneo: NHMUK 1894.7.20.62, NHMUK 1894.21.54-57 (glued on paper).

Other material.

N. Borneo: RMNH/MOL 152748, ZMA/MOLL 315546 (Fig. 5). Gomanton, N. Borneo: MFN 47552, MFN 47942. Kinabatangan valley, Gomantong Hill 30 km South of Sandakan, Sandakan Province, Sabah (05°19.20'N, 118°3.60'E): JJV 1613.

Description.

Protoconch. Colour: white to pale orange, darker than the rest of the shell. Sculpture pattern: irregular sculpture pattern, from base to apex end with no specific sculptural shape. Mesh width: 2.5-8.0 µm. Teleoconch. Colour: white to pale orange. First whorl: flat, convex close to the suture. Subsequent whorls: flat, convex and angular at the periphery. Suture: clearly impressed. Shoulder: narrow. Number of whorls: 2 ¾– 3 ½. SH: 1.91-2.23 mm. SW: 1.65-1.82 mm. SI: 1.12-1.28. Shell sculpture. Radial sculpture: present, weak or flattened, densely sculpted on the whorls, ca. two to three ribs per 0.1 mm. Spiral sculpture: present, but thin and weak, only visible under high magnification (> × 100 magnification), strongest at the first whorl, weaker at subsequent whorls. Aperture. Shape: semi-elliptic, straight parietal side, palatal edge contiguous with the body whorl, palatal side tilted and angular, basal side convex. AH: 0.82-1.07 mm. AW: 1.00-1.09 mm. AI: 0.75-1.02.

Diagnosis.

The flat whorls that are strongly convex at the periphery, giving the shell an angular shape, are diagnostic. The sculpture of G. borneensis resembles that of G. similis and G. corrugata , but is weaker and more flattened than in those species. The spiral sculpture of G. corrugata is also more irregular. The adult shell G. borneensis is larger than in adult G. similis and G. corrugata . Additonally, the base to apex end sculpture of the protoconch of G. borneensis is distinct compared to these species ( G. similis has a rounded protoconch sculpture and G. corrugata has straight-line protoconch sculpture).

Distribution.

The species is known only from Gomantong hill in the Kinabatangan region of Sabah.

Discussion.

The identification of G. borneensis can be confusing when we refer to the sketches by Thompson and Dance (1983: figs 61, 62). These appear to reflect the radial sculpture of G. corrugata , which is wavy/irregular and strongly sculpted. Smith (1895) and Thompson and Dance (1983) described G. borneensis referring to the absence of spiral sculpture, which, however, is present but only visible under high magnification. The bright red colour of the peristome as described by Smith (1895) could not be observed by us, probably due to the faded condition of the shells. We also find there is no association based on colouration of G. borneensis with G. pyrrhoderma and the “williamsi”. Thompson and Dance (1983) grouped the ‘borneensis’ based on their reddish shell colour, while ‘williamsi’ with their light brown colour, of which we find these colours are often a variation within these groups of species.