The first indication--apart from certain disputed impressions in the Devonian--of the land-vertebrate is the footprint of an Amphibian on an early Carboniferous mud-flat. Hardened by the sun, and then covered with a fresh deposit when it sank beneath the waters, it remains to-day to witness the arrival of the five-toed quadruped who was to rule the earth. As the period proceeds, remains are found in great abundance, and we see that there must have been a vast and varied population of the Amphibia on the shores of the Carboniferous lagoons and swamps. There were at least twenty genera of them living in what is now the island of Britain, and was then part of the British-Scandinavian continent. Some of them were short and stumpy creatures, a few inches long, with weak limbs and short tails, and broad, crescent-shaped heads, their bodies clothed in the fine scaly armour of their fish-ancestor (the Branchiosaurs). Some (the Aistopods) were long, snake-like creatures, with shrunken limbs and bodies drawn out until, in some cases, the backbone had 150 vertebrae. They seem to have taken to the thickets, in the growing competition, as the serpents did later, and lost the use of their limbs, which would be merely an encumbrance in winding among the roots and branches. Some (the Microsaurs) were agile little salamander-like organisms, with strong, bony frames and relatively long and useful legs; they look as if they may even have climbed the trees in pursuit of snails and insects. A fourth and more formidable sub-order, the Labyrinthodonts--which take their name from the labyrinthine folds of the enamel in their strong teeth--were commonly several feet in length. Some of them attained a length of seven or eight feet, and had plates of bone over their heads and bellies, while the jaws in their enormous heads were loaded with their strong, labyrinthine teeth. Life on land was becoming as eventful and stimulating as life in the waters.