Saddle Oyster 3-4mm
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I think you could be right here John, Ive seen white versions of this same thing on various things that have come out of the sea including lobsters. When attached totally flat to their home, they dont appear to be bivalves at first.
John you may have already seen this info here, but here it is in case you havent yet:
If you have any doubt over what they are, take one of the rock, if its a saddle oyster there will be a small hole on the underside valve which is hidden, it attaches to the surface where it lives with a foot which comes through the hole.
Your one is quite richly coloured compared to those Ive seen which are very white, but it looks identical otherwise.
Lat/Lng: 54.6455, -1.1375
OS grid ref: NZ557281