I’m puzzling over this shell at the moment. If I recall correctly, it is a live specimen that rolled up the beach with the tide a bit north of Cairns itself, at Yawarra Beach. It looked fairly ordinary and plain until I turned it round to view the edge and saw beautiful delicate growth rings and lovely purple tinged beaks or umbones. I think it is a Mactra. Possibly Mactra dissimilis Reeve.
This group of shells is fairly large with triangular shells; and the animals live in sand. M. dissimilis is the most common species of this family in northern Queensland. It has a sculpture of concentric growth rings but overall is generally smooth in appearance. It is white and tinged with purple and is about 50 mm long. It is found from the Northern Territory to northern New South Wales.
Reference
Jansen, P. (1996) Common Seashells of Coastal Northern Queensland, privately published in Townsville, Australia, ISBN 0 646 29824 0.
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Jessica's Nature Blog 
The closest local species is Mercinaria mercinaria, the hard shelled clam or quahog. The growth lines near of the hinge of the local shells have more of a curve to them. As ever, thank you!
God Bless,
Don
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Your quahog is a really big clam, and one that I am familiar with even here in Dorset. The species was accidentally introduced way back by cruise liners sailing across from America to Southampton in the UK in the 19th century or early 20th. A few specimens were probably thrown overboard with kitchen refuse and managed to survive and form lasting colonies off our south coast.
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The symmetry in your first image is remarkable. Isn’t mother nature amazing.
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It is a strange phenomenon of seashells that they can be symmetrical, asymmetrical or both in the same shell – now that is truly amazing.
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That’s a wonderful find, Jessica, whatever the name of it!
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Thanks RH. Someone is bound to tell me what it is sooner or later.
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