Nummulites is a genus of foraminifera - forams being single-celled amoeboid protists, that build a complex shell and eat bacteria and anything else smaller than them. Quite a few species have endosymbiont algae living inside them, and photosynthesise. They're found in oceans and estuaries worldwide, a few in freshwater, and one species lives in damp rainforest soil. The species I did my degree on, Ammonia beccarri, turns up in estuaries worldwide, and even in a hot spring near the Dead Sea. Some species are planktonic, drifting with ocean currents, and others live on the seafloor, or cling to plants.
They're incredibly important in geology, climate and evolutionary studies, partly because they're highly responsive to changes in temperature and salinity (and the isotopes they build into their calcite tests duly changes), and partly because oil exploration has given us a superb fossil record for them. We can watch them as they evolve, from century to century. Floating them out of the sediment is a protracted job, however, and a fume cupboard is vital unless you like breathing trichloroethane.
Nummulites itself is a large for a protozoan - up to six inches across. This isn't the biggest protozoan - there are other forams that build delicate sponge-like or tetrahedral structures on the deep-sea floor up to 20 centimetres across, and others that roll around menacing tiny Indiana Joneses, but Nummulites more than makes up for it in sheer abundance. Limestone deposits, especially around the Mediterranean, are full of them - the Pyramids are built from nummulitic limestone ( legend has it they're the preserved remains of lentils from the slaves meals - with that amount of supposed beans on site I'm surprised no-one has proposed the Pyramids were built with jet propulsion ).
Nummulites do enjoy another claim to fame - they were the star of the book,The Nummulosphere: an account of the Organic Origin of so-called Igneous Rocks and Abyssal Red Clays by Randolph Kirkpatrick who was the assistant keeper of lower invertebrates at the Museum of Natural History, in London, for some 40 years, and went slightly bonkers. I can sympathise - I spent a mere year staring down the microscope at Ammonia beccarri and could feel my sanity slipping even then - but Kirkpatrick's book tried to prove that all rocks, including granite, were in fact made from fossilised nummulites.
So, forams are certainly important, and Nummulites certainly worthy of respect, but not, alas, as important as that.