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Hyotissa numisma (Lamarck, 1819)

Description: Shell always severely eroded. Shape subcircular, weakly to moderately inflated. Hinge moderately broad. Interior varies from completely white to white with purple-brown muscle scar, to most of the interior purple-brown. Exterior surface irregularly shaped, eroded, chalky. Shell colour obscured by marine growth and erosion.

Size: Up to 70 mm in height.

Distribution: Indo-West Pacific. In Australia, from north-west WA, around northern Australia, to Sydney, NSW.

Habitat: Lives on and under rocks and coral intertidally.

Comparison: This species is characterised by its lack of shell features. It is an irregularly circular flat oyster, not much inflated with eroded surface. The surface is always covered with marine growth and eroded so that no surface ribbing or colour is apparent. Internally, the characteristic honeycombed microsculture is always present.

Synonymy: Ostrea procles Iredale, 1939 is a synonym. Hyotissa numisma might be a smooth-shelled, featureless form of H. inermis (G.B. Sowerby II, 1871).

Fig. 1: Black Head, N. of Richmond River, NSW (C.114578)


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