During the present or Cenozoic era vegetation gradually took on a more and more modern aspect until the existing species were developed. The grasses especially developed and spread rapidly, but the cereals did not evolve until late in the era. Certain single-celled plants, called diatoms, may be especially mentioned, for they must have literally swarmed in some of the Tertiary seas which spread over parts of the present lands. "The microscopic plants which form siliceous shells, called diatoms, make extensive deposits in some places. One stratum near Richmond, Virginia, is thirty feet thick and is many miles in extent; another, near Monterey, California, is fifty feet thick, and the material is as white and fine as chalk, which it resembles in appearance; another, near Bilin, in Bohemia, is fourteen feet thick.... Ehrenberg has calculated that a cubic inch of the fine, earthy rock contains about forty-one thousand millions of organisms. Such accumulations of diatoms are made both in fresh waters and salt, and in those of the ocean at all depths." (J. D. Dana.) CHAPTER XVIII GEOLOGICAL HISTORY OF ANIMALS (EXCLUDING VERTEBRATES) A study of the animals of the past is not only of great interest in itself, but also it furnishes a mainstay of the great doctrine of organic evolution. At the very outset of our discussion the reader should have already in mind at least the main subdivisions of the animal kingdom in order to reasonably well understand where the important animal types of the different geological ages fit in, and how those types bear upon the doctrine of evolution. The accompanying, very brief, general classification includes the usually recognized subkingdoms with special reference to representatives of those which are of most geological and evolutionary significance. Reading downward in this classification, the degree of complexity of organization steadily increases from single-celled animals to man himself. I. Protozoans, e.g. foraminifers (with lime carbonate shells) {Sponges II. Coelenterates, e.g. {So-called "jellyfishes," graptolites {Corals {So-called "sea lilies" III. Echinoderms, e.g. {So-called "starfishes" {So-called "sea urchins" IV. Worms, V. Molluscoids, e.g. {So-called "sea mosses" {Brachiopods {Clams, oysters { {Snails {Pearly nautilus, VI. Mollusks, e.g. { { ammonites { {Cephalopods, e.g. {So-called { { "cuttle fishes"