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Onoba (Subestea) australiae (Frauenfeld, 1867)

Description; Shell moderately tall, spire convex. Protoconch of about 1 whorl with about 4 coarse spiral cords, ending in distinct junction to teleoconch. Teleoconch up to 4 moderately rounded whorls. Sculpture variable in strength; axial ribs strongest in middle of whorls, fading out at top and bottom of whorls, and at periphery of last whorl; about 14 ribs on second-last whorl. Spiral threads weaker than axial ribs; usually 2 on first teleoconch whorl, increasing to 4-5 above periphery on last whorl; about 8 more spirals on base of last whorl. Aperture ovate, weakly channelled posteriorly; outer lip expanded and thickened externally with varix. Umbilicus closed. Shell translucent colourless fawn or brown when fresh, becoming opaque with age.

Size: Up to 2.5 mm in length.

Distribution: Endemic to Australia: Port Stephens, NSW, southwards to Tasmania.

Habitat: Empty shells known from beach washup and down to 80 m. Living specimens from intertidal algal and rock washings. Moderately common.

Synonymy: Previously recognised synonyms are Rissoia ochroleuca Brazier, 1894; Rissoa mixta Tate, 1899; Rissoina apicilirata Tate & May, 1900; Alvania seminodosa May, 1915; .

Remarks: Shells are variable in size and sculpture, giving rise to the synonyms listed above. Differences in the number of whorls at maturity have been seen within a single lot, with mature specimens of 3 whorls being 1.8 mm long, and of 4 whorls being 2.5 mm long. The axial ribs are variable in strength between specimens, from strong and nodulose to weak and smooth, and sometimes fade out on the last whorl, particularly on the last quarter whorl.

Fig. 1: Batehaven, Batemans Bay, NSW (C.347973)

 

Copyright Des Beechey 2010