Jurassic Period, circa 160 million years B.P. A polished fossil Rastellum carinatum bivalve shell; held in a card collector's tray. 128 grams, 76 mm (3 in.). From Madagascar. From a Cambridgeshire, UK, collection. This bivalve is from the oyster family and 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 and fed on food particles that it filtered out of the sea water. [No Reserve]