With all these circumstances in its favor, warmth, moisture, and an atmosphere charged heavily with carbon, vegetation could not help but flourish. That it did flourish amazingly is abundantly shown by its fossil remains. The impressions of more than five hundred different species of plants that grew in the Carboniferous era have been found in the coal measures. There are few of them that bear any direct analogy to existing species, and these few have their counterparts only in the torrid zone. The most abundant of the plants of the coal era were the ferns. Their fossil remains are found in great profusion and variety in most of the rocks of the coal-bearing strata. There was also the plant known as the tree fern, which attained a height of twenty or thirty feet and carried a single tuft of leaves radiating from its top. Probably the species next in abundance, as it certainly is next in importance, to the ferns is that of the Lepidodendrids. It doubtless contributed the greatest proportion of woody material to the composition of coal. The plants of this species were forest trees, but are supposed to have been analogous to the low club mosses of the present. Fossil trunks of Lepidodendrids have been found measuring from one hundred to one hundred and thirty feet in length, and from six to ten feet in diameter. Similar in appearance to the Lepidodendrids were the Sigillariæ, which were also very abundant. The Conifers were of quite a different species from those already named, and probably grew on higher ground. They were somewhat analogous to the modern pine. The Calamites belonged to the horsetail family. They grew up with long, reed-like, articulated stems to a height of twenty feet or more, and with a diameter of ten or twelve inches. They stood close together in the muddy ground, forming an almost impenetrable thicket, and probably made up a very large percentage of the vegetation which was transformed into coal. One of the most abundant species of plants of the coal era is that of Stigmaria. Stout stems, from two to four inches in diameter, branched downward from a short trunk, and then grew out in long root-like processes, floating in the water or trailing on the mud to distances of twenty or thirty feet. These are the roots with which the under clay of every coal seam is usually filled. The plants which have been described, together with their kindred species, formed the largest and most important part of the vegetation of the Carboniferous age. But of the hundreds of varieties which then abounded, the greater portion reached their highest stage of perfection in the coal era, and became extinct before the close of Paleozoic time. Other types were lost during Mesozoic time, and to-day there is scarcely a counterpart in existence of any of the multitude of forms of plant life that grew and flourished in that far-off age of the world.