Exhibition hall: Diversity of Life Slit Shells: “Living Fossils” Display Location: The Biodiversity Room These shells are called “millionaire shells” (Chouja-gai) in Japanese and were featured on the old four yen stamp. This name comes from the story of Kumakichi Aoki, who earned a reward from the Natural History Museum, London by finding a pristine specimen of the rare shell. Slit shells appeared in the Paleozoic Era and have changed very little in all that time, so they are also known as living-fossils.