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It can be nicely observed how the complete shell was infilled at the bottom with mud during deposition and subsequently calcite crystals formed after deposition or diagenesis. It is a wonderful example of how to tell palaeo-up from down especially in tilted sediments. The complete preservation of the shell leads me to assume that it was deposited either in-situ or without much transport. Long transport or strong wave and current action would have seperated the two shell parts.