The remains of these Diatomaceae are of pure silex, and their forms are various, but very marked and constant in particular genera and species. Thus, in the family _Bacillaria_ (see fig. 16.), the fossils preserved in tripoli are seen to exhibit the same divisions and transverse lines which characterize the living species of kindred form. With these, also, the siliceous spiculae or internal supports of the freshwater sponge, or _Spongilla_ of Lamarck, are sometimes intermingled (see the needle-shaped bodies in fig. 20.). These flinty cases and spiculae, although hard, are very fragile, breaking like glass, and are therefore admirably adapted, when rubbed, for wearing down into a fine powder fit for polishing the surface of metals. Besides the tripoli, formed exclusively of the fossils above described, there occurs in the upper part of the great stratum at Bilin another heavier and more compact stone, a kind of semi-opal, in which innumerable parts of Diatomaceae and spiculae of the _Spongilla_ are filled with, and cemented together by, siliceous matter. It is supposed that the siliceous remains of the most delicate Diatomaceae have been dissolved by water, and have thus given rise to this opal in which the more durable fossils are preserved like insects in amber. This opinion is confirmed by the fact that the organic bodies decrease in number and sharpness of outline in proportion as the opaline cement increases in quantity. In the Bohemian tripoli above described, as in that of Planitz in Saxony, the species of Diatomaceae (or Infusoria, as termed by Ehrenberg) are freshwater; but in other countries, as in the tripoli of the Isle of France, they are of marine species, and they all belong to formations of the _tertiary_ period, which will be spoken of hereafter. A well-known substance, called bog-iron ore, often met with in peat-mosses, has also been shown by Ehrenberg to consist of innumerable articulated threads, of a yellow ochre colour, composed partly of flint and partly of oxide of iron. These threads are the cases of a minute microscopic body, called _Gaillonella ferruginea_ (fig. 18.). [4 Illustrations: _Cytheridae_ and _Foraminifera_ from the chalk. Fig. 21. _Cythere_, Muell. _Cytherina_, Lam. Fig. 22. Portion of _Nodosaria_. Fig. 23. _Cristellaria rotulata._ Fig. 24. _Rosalina._]