This is part of the underside of a turtle shell, known as the plastron. The top side is the carapace. Turtle shells are made up of a series of plates that link together to form the overall structure which is supported by an internal skeleton of vertebrae and ribs. This is the right, rear or possibly front, section of the plastron and there would have been an identical plate mirroring it below. The indentation in the top of the plate, as photographed, would be for the leg
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