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RISSOIDAE


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Rissoina (Rissoina) cretacea Tenison-Woods, 1878

Description: Shell with spire slightly convex. Protoconch of 1 whorls, with distinct junction with teleoconch. Teleoconch of 5 - 6 weakly convex whorls, weakly angled at top. Whorls sculptured with weak, irregularly spaced axial ribs, weaker on bottom of whorls, fading out on latter whorls; on last whorl present as weak folds on top third of whorl. Spiral sculpture absent. Aperture lens or D-shaped, with broad anterior canal and narrow posterior canal; outer lip expanded, slightly thickened but smooth internally, with minute varix externally; inner lip callused, thickened at top and bottom. Colour uniformly white or yellowish, polished and shining, translucent when fresh, becoming opaque with age.

Size: Up to 6.8 mm in length.

Distribution: Endemic to Australia; Cape Moreton, Queensland, southwards to Gabo Island, Victoria.

Habitat: Subtidal, in 27-120 m. Moderately common.

Comparison: R. cretacea is similar to R. rhyllensis, both having fairly smooth shells, and occurring subtidally. R. rhyllensis has a relatively large protoconch of 2 whorls, while that of R. cretacea is small, of 1 whorls. The whorls of R. cretacea are more convex than those of R. rhyllensis, and the aperture is narrower.

Synonymy: None.

Fig. 1:  Off Gabo Island, Victoria, 27 m (C.135037)

 

Copyright Des Beechey 2010