any of the thousands of extinct species of relatively large, lens-shaped foraminifers (single-celled marine organisms) that were abundant during the Tertiary Period (66.4 to 1.6 million years ago). Nummulites were particularly prominent during the Eocene Epoch (57.8 to 36.6 million years ago), and limestone of this age that occurs in the Sahara is called nummulite limestone in reference to the great abundance of its contained fossil nummulites.