There are three other regions in England and Scotland where the Old Red Sandstone is conspicuous, and all of them were probably old fresh-water lakes of great extent in which sands accumulated. Sir Archibald Geikie has named them the Welsh Lake, Lake Cheviot, Lake Caledonia, Lake Arcadie, Lake Lorne. There are similar sandy deposits in Russia, in North America, near the Catskill Mountains, and in many parts of Canada; and there is little doubt that in all these places there were great lakes which gradually became the depositories of rivers and developed a life of their own. The most remarkable fossils of these deposits were the fishes. The fishes began to appear in the later Silurian: they are strikingly abundant in Devonian times. The most remarkable of them are fishes which are only just like fishes after having been developed out of, or perhaps descending from, some other form of life. These fishes are now called the _Ostracoderm_ group, and they bear strange resemblance to some of the trilobites and the king crabs of previous eras. The _Pteraspis_ is one of the earliest of these strange creatures, and its "fins," very much developed, were used as oars. Perhaps the most curious of all these strange creatures were the _Pterichthyds_ or winged fish; though it is not at all likely that the appendages we call wings were used for aerial flight. These fishes were all small; their forms were clumsy and their powers of moving about small. They had poor mouths and eyes, and they probably ploughed the soft bottoms of the sluggish waters, above which little besides their peculiarly placed eyes and the backs of their plated bucklers were habitually exposed. Another strange class of fish-like creatures was represented by a little creature which was found in Scotland and is sometimes supposed to be the ancestor of the lamprey. Besides the fresh-water fishes there were some which dwelt in the sea; but in the Devonian era the fresh-water fishes were far more numerous. We cannot mention them all. The fish called _Coccosteus_ and its allies had great bony plates of considerable thickness on its head and shoulders (some fine examples are to be seen at the Natural History Museum, Cromwell Road, London), but its tail and middle body were left unprotected. The sharks of to-day had their representatives among the Devonian fishes. Sharks have throughout geological time nearly always been sea-dwellers, though they still occasionally live in fresh water, as in Lake Baikal in Siberia and Lake Nicaragua. It seems clear, however, that in the Devonian period they lived in the open sea. But their remains are found in the Old Red Sandstone, and therefore it is likely that they lived in fresh and brackish waters also. In the same strata as these remarkable fishes there are found some large and peculiar crustaceans, something like our modern king crabs, but reaching the enormous length of six feet. There have also been mussels found and a few water plants, but not many.