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RISSOIDAE

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Rissoina (Rissoina) decipiens (Laseron, 1956)

Description: Shell with spire moderately convex. Protoconch of 2 whorl, with ridged sinusigeral junction with teleoconch. Teleoconch of 7-8 weakly convex whorls, shouldered on early whorls, rounded on later whorls. Whorls sculptured with strong axial ribs; early spire whorls with weak spiral threads. Last whorl with very strong, nodulose spiral rib on base with a wide, deep groove above. Aperture lens or D-shaped, with broad anterior canal and narrow posterior canal; outer lip expanded, not thickened internally, with varix externally; inner lip callused, thickened at top and bottom. Colour uniformly white, translucent when fresh, becoming opaque with age.

Size: Up to 7.4 mm in length.

Distribution: Endemic to Australia; Cairns, Queensland, southwards to Sydney, NSW.

Habitat: Known mainly from empty shells from beach washup. Moderately common.

Comparison: This species is almost identical in shape to R. crassa and is very similar to R. angasi. It differs from both by having a protoconch of 2 whorls ending with a sinusigeral notch with a strongly raised edge (Fig. 2), rather than 1 whorl with simple margin. The early spire whorls show fine spiral threads that are absent in R. crassa and R. angasi.

Synonymy: None.

Fig. 1: Mona Vale, NSW (DLB5388)