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Amphithalamus incidatus (Frauenfeld, 1867)

Description: Shell minute, solid, spire weakly to moderately convex in outline. Protoconch paucispiral, rounded, with spiral rows of minute pits, with strong varix forming distinct junction with teleoconch. Teleoconch whorls weakly rounded, without axial or spiral sculpture; last whorl weakly angled or with low, wide cord or groove at periphery on ventral side, fading out on last quarter whorl. Aperture ovate, with no anterior or posterior canals. Inner lip of aperture separated from previous whorl by narrow channel, with a callus plug at suture. Outer lip without varix externally, thickened but smooth internally. Umbilicus closed. Colour fawn to reddish-brown.

Size: Up to 1.5 mm in length.

Distribution: Endemic to Australia: Caloundra, Queensland, southwards and around southern Australia, to south-western WA, including Tasmania.

Habitat: Living specimens are common to frequent in algae from the low intertidal down to at least 28 m. Empty shells are common in beach washup and down to 113 m.

Comparison: Comparison of the species of Amphithalamus: A. incidatus and A. pyramis are very similar. A. pyramis reaches 2.0 mm and A. incidatus reaches 1.5 mm, but there are often small specimens of the former. The spire outline of A pyramis varies from almost straight to weakly convex, and of A. incidatus from weakly to moderately convex. Both species may or may not have a ridge or groove on the periphery. The most consistent difference is the width of the channel between the inner lip and the previous whorl - consistently wide in A. pyramis, as against consistently narrow in A. incidatus. Amphithalamus jacksoni is quite distinctive in shape, with the widest point of the shell at mid body. The channel between the aperture and the previous whorl is very wide.

Synonymy: None.

Fig. 1: Reef Bay, North Harbour, Sydney, NSW (C.351785)


Copyright Des Beechey 2010, modifed 2014, modified 2016