(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.