C A flat plate made of glass (or plastic etc.) is called "petri dish", but the bottom of this product is not flat. That is not a flat plate. There seems to be a thick part of meat on the bottom during the manufacturing process, there is a part raised like a lens in a part of the bottom. Still other than the outer periphery of the bottom is recessed slightly so that it can be placed stably, so the above lens part is further raised inside.