Found this odd shell in a garden (where it was obviously being used ornamentally). Seemed very large. Any idea what it once was?
To give an idea of scale the lens-cap shown is about 65mm in diameter.
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Is it collected for ornamental purposes (as this one was being used) or for food?
Apparently it was a favourite ingredient in Ceviche, a raw sea food dish from Central and South America. It seems from this website that it may now be extinct (http://research.calacademy.org/ccg/staff/bsimison). Also, its correct scientific name should be Scutellastra mexicana (http://www.marinespecies.org/aphia.php?p=taxdetails&id=456684) so I have added a new identification to reflect this.
Lat/Lng: 60.2417, -1.3516
OS grid ref: HU360510