☞ The rhizopods belonging to the Radiolaria and Foraminifera have been of great geological importance, especially in the Cretaceous and Tertiary periods. Chalk is mostly made from the shells of Foraminifera. The nummulites are the principal ingredient of a limestone which is of great extent in Europe and Asia, and is the material of which some of the pyramids of Egypt are made. The shells are abundant in deepsea mud, and are mostly minute, seldom larger than a small grain of sand, except in the case of the nummulities, which are sometimes an inch in diameter.