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Cylichna thetidis Hedley, 1903

Description: Shell elongate, involute, width about 36% of total shell length. Apex with narrow umbilicus surrounded by a ridge contiguous with outer lip. Last whorl with sides very weakly convex over whole length of shell. Aperture narrow over top 2/3 of shell, wide at bottom; outer lip sharp. Columella with moderately strong plait, with narrow umbilicus behind, sometimes covered by expanded callus. Parietal wall with thin white callus which extends posteriorly to partly obscure apical umbilicus; callus often peeled away. Whole surface with dense microscopic spiral grooves, weaker or absent in middle. Shell white or pale fawn rarely with thin brown periostracum, worn away on most specimens.

Size: Up to 14 mm in length.

Distribution: Australia and northern New Zealand. In Australia, Coffs Harbour, NSW, southwards and around southern Australia to southwestern WA, including Tas. Also Lord Howe Island and Norfolk Island.

Habitat: Usually subtidal, only occasionally in beach washup. Most frequently dredged in 10-100 m, occasionally down to 300 m. Common.

Comparison: This species is similar to Adamnestia arachis but is smaller and thinner and slightly more narrow and elongate. At the apex there is a more or less well defined ridge on top of the whorls, which is absent in A. arachis. The columella has a moderately strong fold. This is a subtidal species, rarely found in beach washup, where A. arachis is common.

Figs. 1, 2:  2-4 miles (3-6 km) off Manning River, NSW, in 22 fathoms (40 m) (C. 16488 HOLOTYPE). (Photo Australian Museum).


Copyright Des Beechey 2018