The Calamites, growing in vast fields along the margins of the Sigillaria forests, must have greatly protected these from the effects of inundations, and by collecting the mud brought down by streams in times of flood, must have done much to prevent the intrusion of earthy deposits among the vegetable matter. Their chief office, therefore, as coal-producers, seems to have been to form for the Sigillaria forests those reedy fringes which, when inundations took place, would exclude mud, and prevent that mixture of earthy matter in the coal which would have rendered it too impure for use. Quantities of fragments of their stems can, however, be detected by the microscope in most coals. The modern Mares' Tails have thin-walled hollow stems, and some of the gigantic calamites of the coal resembled them in this. But others, to which the name _Calamodendron_, or Reed-tree, has been given, had stems with thick woody walls of a remarkable structure, which, while similar in plan to that of the Mares' Tails, was much more perfect in its development. Professor Williamson has shown that there were forms intervening between these extremes; and thus in the calamites and calamodendrons we have another example of the exaltation in ancient times of a type now of humble structure; or, in other words, of a comprehensive type, low in the modern world, but in older periods taking to itself by anticipation the properties afterward confined to higher forms. The gigantic club-mosses of the Coal period constitute a similar example, and it is very curious that both of these types have been degraded in the modern world, though retaining precisely their general aspect, while the tree-ferns contemporary with them in the Palæozoic still survive in all their original grandeur.