, 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. (Original.)]

[Illustration: Fig. 16.--Cases of Diatoms in the Richmond "Infusorial
earth;" highly magnified. (Original.)]

In addition to flint-producing animals, we have also the great
group of fresh-water and marine microscopic plants known as
_Diatoms_, which likewise secrete a siliceous skeleton, often of
great beauty. The skeletons of Diatoms are found abundantly at the
present day in lake-deposits, guano, the silt of estuaries, and in
the mud which covers many parts of the sea-bottom; they have been
detected in strata of great age; and in spite of their microscopic
dimensions, they have not uncommonly accumulated to form deposits
of great thickness, and of considerable superficial extent. Thus
the celebrated deposit of "tripoli" ("Polir-schiefer") of Bohemia,
largely worked as polishing-powder, is composed wholly, or almost
wholly, of the flinty cases of Diatoms, of which it is calculated
that no less than forty-one thousand millions go to make up a
single cubic inch of the stone. Another celebrated deposit is
the so-called "Infusorial earth" of Richmond in Virginia, where
there is a stratum in places thirty feet thick, composed almost
entirely of the microscopic shells of Diatoms.

Nodules or layers of _flint_, or the impure variety of flint
known as _chert_, are found in limestones of almost all ages
from the Silurian upwards; but they are especially abundant in
the chalk. When these flints are examined in thin and transparent
slices under the microscope, or in polished sections, they are
found to contain an abundance of minute organic bodies--such as
_Forami