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Isognomon isognomum (Linnaeus, 1758)

Description: Shape circular when juvenile (up to 30 mm), becoming markedly elongate with growth; valves flattened. Dorsal margin straight, anterior margin with byssal notch in right valve. Hinge line straight with numerous ligamental blocks, number increasing with shell growth. Interior nacreous, with narrow, brown non-nacreous margin; single adductor muscle scar centrally. Exterior with irregular concentric laminae, usually eroded. Shell colour creamish brown to pale purple, sometimes with light coloured radiating rays.

Size: Up to 200 mm in height.

Distribution: Indo-West Pacific. In Australia, from North West Cape, WA, around northern Australia and southwards to Botany Bay, Sydney, NSW.

Habitat: This species lives intertidally and in the shallow subtidal, attached to solid substrates by the byssus, with the shell positioned vertically. Yonge (1968) reported on a dense bed of this species intermixed with Malleus regulus at Palau Gaya, Philippines.

Synonymy: The specific name of this species has often been incorrectly spelt as isognomon.

Fig. 1:  Angourie Point, NSW (C.072067)


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