Amongst the animals which require special mention in this connection are the microscopic organisms which are known to the naturalist as _Polycystina_.

These little creatures are of the lowest possible grade of organisation, very closely related to the animals which we have previously spoken of as _Foraminifera_, but differing in the fact that they secrete a shell or skeleton composed of flint instead of lime.

The _Polycystina_ occur abundantly in our present seas; and their shells are present in some numbers in the ooze which is found at great depths in the Atlantic and Pacific oceans, being easily recognised by their exquisite shape, their glassy transparency, the general presence of longer or shorter spines, and the sieve-like perforations in the walls.
Both in Barbadoes and in the Nicobar islands occur geological formations which are composed of the flinty skeletons of these microscopic animals; the deposit in the former locality attaining a great thickness, and having been long known to workers with the microscope under the name of "Barbadoes earth" (fig.

15).
[Illustration: Fig.

15 .-- Shells of _Polycystina_ from "Barbadoes earth;" greatly magnified.