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Family  Isognomonidae

Toothed Pearl Shells




The Isognomidae is a numerically small family of oysters characterised by an unusual ligament. There are less than 20 species in the family. In overall shell shape isognomonids are similar to the Malleidae, but the hinge has a completely different structure; the hinge line is straight, as in the Malleidae, but there are short perpendicular ligamental blocks, the number of which increase with growth of the shell. This distinctive structure is known as a multivincular ligament.

Isognomids are generally tropical, distributed throughout the Indo-West Pacific as well as the Caribbean and eastern Atlantic. They live intertidally or in the shallow subtidal, attached firmly by a thick byssus to solid substrates such as rocks or mangrove roots, sometimes occurring in dense aggregations. Aborigines in northern Australia harvested Isognomon ephippium for food (Bourke et al. 2023).

The two species that occur in NSW are of Indo-West Pacific distribution, and restricted to the north of the state. They are uncommon in northern NSW and rarely reach as far south as Sydney.

Family Reference

The NSW species are among those described in detail by Yonge (1968).


Only two species are known from NSW; both are detailed here.

Identification Notes

The characteristic hinge with numerous vertical ligamental blocks readily places species in this family.


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