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RISSOIDAE


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Rissoina (Rissoina) crassa Angas, 1871

Description: Shell with spire moderately convex. Protoconch of 1 whorl, with distinct junction with teleoconch. Teleoconch of 6-7 weakly convex whorls, shouldered on early whorls, rounded on later whorls. Whorls sculptured with strong axial ribs, without spiral sculpture. 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 strong varix externally; inner lip callused, thickened at top and bottom. Colour uniformly white, translucent when fresh, becoming opaque with age.

Size: Up to 8.0 mm in length.

Distribution: The specimens identified as this species in the Australian Museum collection cover a range from Yamba, NSW, southwards to Sydney, and Sleurs (1992) restricts it to NSW. But earlier sources (Macpherson & Gabriel, 1962; Cotton, 1959) record it from Victoria and South Australia. This may reflect different interpretations of R. crassa and R. angasi, or the extended range may be real.

Habitat: Most specimens are empty shells from beach washup, but several are subtidal down to 18 m. Moderately common.

Comparison: This species is very similar to Rissoina angasi but has a much wider and heavier shell. The early whorls are angled but  the last and second-last whorls are rounded., in contrast to R. angasi which has all whorls angled. It is also similar to Rissoina decipiens Laseron, 1956, but lacks the fine spiral sculpture on the early whorls of that species. It has a protoconch of 1 whorl with a simple edged junction with the teleoconch (Fig. 2) in contrast to R. decipiens which has a 2 whorled protoconch with a sinuous, raised ridge at the junction.

Synonymy: None

Fig. 1: Woody Head, near Iluka, NSW (C.349110)

Fig. 2: Woody Head, Iluika, NSW (C.396015). SEM Image Sue Lindsay, Australian Museum

Copyright Des Beechey 2010 Modified 2014