Every one who has been on a chalk cliff or hill has found, and perhaps thrown, chalk flints. Flints are made of mineral called silica, and very often these flints, or nodules of silica, surround some organism like a sponge or a shell. During the formation of the chalk the sea floor appears to have been covered at intervals by a growth of sponges, which were composed of siliceous matter, and their death and decay produced most of the flint. Sometimes flint is found in bands, in which case it may have been deposited by siliceous water trickling through fissures or cracks in the chalk. In the sea which thus existed the Plesiosaurs and Ichthyosaurs still pursued the even tenor of their way, growing larger and larger. They were of many shapes, and probably of many habits. Some were certainly fish-eaters, and with their enormous jaws must have been most undesirable neighbours. Probably, however, they had plenty of diversity in their lives, and may have had many a bitter struggle with equally ferocious sea animals of other types. The scaly saurians, for example, were beginning to come on; and began in this era to assume the size and appearance that have occasionally since been attributed to sea serpents. These reptiles, known as _Dolichosaurs_, were long-necked, lizard-like reptiles in the beginning of their career, and grew longer and longer in succeeding generations, till at last their descendants were so long and snaky that geologists have called the later specimens "serpents." These sea serpents were from fifteen to forty-five feet in length, and their remains have been found in the valley of the Meuse. They do not seem to have had a very long career, for they do not appear after the Chalk Age, and no direct descendants are known; but while they lived they ranged from North and South America to Europe and New Zealand. The first true sea turtles appeared and lived and extended their families in great variety. They had broad flat forms, their shells only just covering their ribs like a short Eton jacket; but they were very large. The greatest of them, _Archelon_, had a skull larger than that of a horse, and must have measured fully twelve feet across the shell.