Familiar examples of these “unicellular” forms are Bacteria and Diatoms among the plants, and Foraminifera and Infusoria among the animals.

In many cases it is certain that these are casts, which fill up the interior of empty shells of Foraminifera.

The city of Richmond, Virginia, is built over rocks, composed largely of the minute fossils of Diatomace� intermingled with the Foraminifera and others.