Tests of shelled amoebas are made from a variety of substances including sand grains, chitin and substrate, calcium salts, and even silica. These shells still accumulate in ocean depths and are referred to as strews. One of the more famous fossil strews are white cliffs of Dover. Chalk from which they are made are the shells of ancient amoebas. This tells you a little about the diversity and numbers of these single-cell organisms which flourished in oceans so long ago.