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These are Jurassic aged fossil oyster of the species Rastellum carinatum from Tulear Province, Madagascar. It has wide, angled ribs that have led to it being called the 'denture clam'. The zig-zag join between the two shells stopped coarse dirt and debris entering the shell and damaging its soft body. Like modern oysters it lived in shallow coastal waters including the intertidal zone (the area between high and low tide) and fed on food particles that it filtered out of the sea water. Your fossil will be in pristine condition and will be randomly selected from the lot. They average at 3-4 inches.