A genus of unicellular shelled protoctist (protozoa) of the class Foraminifera (order Rotaliida, family Nummulitidae). The shells of Nummulites and other large, internally complex foraminifers commonly occurred in rock-forming abundances on continental shelves and around oceanic islands throughout subtropical and tropical regions during the Cenozoic. Resulting limestones are important hydrocarbon reservoirs from the Philippines and Indonesia westward to Trinidad and Venezuela. Nummulitic limestones have been used as building materials around the Mediterranean; the pyramids of Egypt were constructed of blocks of Eocene age. Nummulites shells are useful stratigraphic and paleoenvironmental indicators; the Paleogene was formerly known as the Nummulitic Period.