There are, however, as we shall see, many other limestones, which are likewise largely made up of _Foraminifera_, but in which the shells are very much more minute, and would hardly be seen at all without the microscope.
[Illustration: Fig.

10 .-- Piece of Nummulitic Limestone from the Great Pyramid.

Of the natural size.

(Original.)] We may, in fact, consider that the great agents in the production of limestones in past ages have been animals belonging to the _Crinoids_, the _Corals_, and the _Foraminifera_.

At the present day, the Crinoids have been nearly extinguished, and the few known survivors seem to have retired to great depths in the ocean; but the two latter still actively carry on the work of lime-making, the former being very largely helped in their operations by certain lime-producing marine plants (_Nullipores_ and _Corallines_).