These strata furnish a great many organic remains, especially the shells of the conchiferous mollusca and cephalopods, as Ammonites (fig. 22), Belemnites, Nautili, and Pentacrinites (fig. 23), of which a great many varieties are found, also the remains of gigantic reptiles, as the Ichthyosaurus (fig. 24), Plesiosaurus (fig. 25), and others. New forms of animal existence seem to have been created in this period in great abundance, and the waters of the earth once again became the theatre of deposition for the shells and polypidomes of zoophytes and molluscs, which swarmed them in myriads, and another great group of rocks began to be formed, namely, the chalk or "Cretaceous" system, which form the cliffs and downs of our south coasts, and strata of great extent in nearly every part of the world; it differs from limestone only in not being so hard, which is supposed to arise from its not having undergone the changes caused by heat and pressure. [Illustration: FIG. 24.--ICHTHYOSAURUS.] [Illustration: FIG. 25.--PLESIOSAURUS.] [Illustration: FIG. 26.--NAUTILUS INEQUALIS.] [Illustration: FIG. 27.--ORGANISMS FROM CHALK.] The chalk is interstratifìed with lines of sand, and the lower part is almost entirely sandy, forming the gault and greensand deposits; these each contain organic remains, and must have been the original sand of the sea-bottom before the chalk was deposited on it. These seas must have been the residence of a vast number of reptiles, for the gault contains an enormous number of nodules of what appears to be stone, but which upon closer examination, prove to be coprolites, or portions of the excrement of those creatures partly fossilised, but still retaining phosphate of lime enough to render them valuable manure, and these--like the coal--after being buried in the earth for thousands of years, are now being brought into use. In these coprolites may be constantly seen the teeth and bones of fishes, together with portions of echini and crustaceans, which had passed through the intestines of these saurian reptiles. Fig. 26 is the shell of a small nautilus (_Nautilus inequalis_) thus found. [Illustration: FIG. 28.--AMMONITE FROM THE CHALK.]