Tree-ferns flourish in warm climates, and are met with in Brazil, the East and West Indies, New Zealand, &c. They rise sometimes to the height of fifty or sixty feet, with a long tapering stem surmounted by a dense crown of graceful fronds, and might easily be mistaken at a little distance for palms. All the known species belong to the same division (_Polypodiaceæ_) with the common polypodium of our road-sides. In some genera, as the _alsophila_ of the East Indies, the trunk is ribbed by long creeping branches, or rather rootlets, which descend to the soil, giving the tree somewhat of the appearance so often seen in old woods, where venerable fir-trees have been firmly encased by the bearded stems of the ivy. Another genus, the _Cyathea_, has its stem covered with oblong scars where leaves were attached, and a circle of rich outspread fronds surmounts its summit. One of the coal-measure tree-ferns seems to have resembled this recent type. It is named the _Caulopteris_ or stalk-fern, and had a thick stem picturesquely roughened by irregular oblong leaf-scars, that wound spirally from its base to its point. No specimen has hitherto been found showing the fronds in connexion with the stem, so that we are still ignorant of the kind of foliage exhibited by this ancient tree. There can be no doubt, however, that it was crowned with a large tuft of boughs that cast their shadow over the sward below, and we may, perhaps, believe that some of the numerous detached ferns found in the shales of the coal-series, once formed part of this lofty coronal.