The circle of animal life was completed in this epoch, thus far, that every class was represented by some one or more of its constituent species. No fossil skeletons of Birds have, indeed, been found so low as the Oolite, but numerous foot-prints of some of the Grallatores are found in a sandstone of this period; and in the Stonesfield slate, which is contemporary with it, a genus of Mammalia has been discovered,--a small Marsupial, allied to the Opossums of America. The duration of the Oolitic period must have been considerable. "The lias sea-bottom was succeeded first by a sandy, and then by a calcareous deposit, and the animals were modified accordingly." The deposit of carbonate of lime, which took place under circumstances that caused it to attract around its nodules the organic particles, whence the name _oolite_ (egg-stone) is derived, was not continuous, but repeated at intervals. The shells of Mollusks were developed in great abundance, and accumulations of these formed thick bands, which consolidated into layers of shell-limestone. Three hundred feet of strata, largely composed of organic remains, were formed before the clay was deposited which made the Stonesfield and contemporaneous slates. Once more the dry land sank, probably by slow successive subsidences, and the sea flowed many fathoms deep above the great European archipelago. And upon its quiet bottom settled down, first a few sandy and clayey beds, and then the great layer of the Chalk. Creatures of very minute size and low grades of organization were now playing a very important part. A large portion of the lime that was deposited, in the form of a pure carbonate, was doubtless supplied by the Coral structures, which, were exceedingly numerous; the polypidoms being gnawed down by strong-jawed fishes that fed upon the Zoophytes. _Foraminifera_ also were abundant, and contributed to the supply. Nodules of flint exist in the Chalk, sometimes scattered, sometimes arranged in bands. Two sources are indicated for this substance. One is Sponge, the most common kinds of which are composed of skeletons of siliceous spicula; and these can be discerned with the microscope in the interior of the chalk-flints. But millions upon millions of Infusoria swam through the waters, and many of these were encased in siliceous loricae, while the rocks and sea-weeds were fringed with as incalculably numerous examples of siliceous _Diatomaceae_, whose elegant forms are recognisable without difficulty throughout the Chalk. The inconceivable abundance of these forms may be illustrated by the often-cited fact, that whole strata of solid rock appear to be so exclusively composed of their solid remains, that a cube of one-tenth of an inch is computed by Ehrenberg to contain five hundred millions of individuals.