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Rissoina (Rissoina) ambigua (Gould, 1849)

Description: Shell variable in size, shape and sculpture; spire convex. Protoconch of 2 whorls, with well-defined junction to teleoconch with sinusigeral notch. Teleoconch of about 7 weakly convex whorls, weakly shouldered below deeply impressed suture. Sculpture of narrow axial ribs, 20-35 on last whorl, covering the whole surface, and extending down to inner lip of aperture on last whorl; spiral sculpture of microscopic threads between axials (often eroded away), stronger on base of last whorl. Aperture lens or D-shaped, with broad anterior canal and narrow posterior canal; outer lip greatly thickened externally; inner lip callused, thickened at top and bottom. Colour white, sometimes with brown spiral band.

Size: Up to 7.7 mm in length.

Distribution: Tropical Indo-West Pacific; in Australia, Shark Bay, WA, around northern Australia, to Sydney.

Habitat: Intertidal. Common in the tropics, rare in NSW.

Comparison: This species has some resemblance to finely ribbed forms of R. fasciata, but the axial ribs on the last whorl extend over the base of the shell, down to the lip callus, while in R. fasciata the axials are much coarser and fade out at the periphery on the last whorl.

Synonymy: Synonyms given by Sleurs (1992) include Rissoina pulchella Brazier, 1877; Peripetella immersa Laseron, 1956; Peripetella queenslandica Laseron, 1956

Fig. 1: Groper Is., Burdekin River mouth, Queensland (C.312942)


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