In comparing the Calamarieae with the Equisetaceae, the alternation of sterile and fertile whorls in the strobilus, and the free linear leaves at the nodes instead of leaf-sheaths are two characters made use of as distinguishing features of the genus _Calamites_ as the type of the Calamarieae. On the other hand, the strobili of _Phyllotheca_ appear to agree with those of _Calamites_ rather than with those of _Equisetum_, and strobili of _Archaeocalamites_ have been found exhibiting the typical _Equisetum_ characters. The sheath-like form of the leaves is not necessarily peculiar to the Equisetaceae, and we have evidence that leaf-sheaths occurred on the nodes of Calamitean plants. In _Archaeocalamites_ the leaves possess characteristic features, and can hardly be said to agree more closely with those of Calamites than with the leaves of _Phyllotheca_ or _Sphenophyllum_, a genus belonging to another class of Pteridophytes. On the whole, then, without discussing further the possibilities of a subdivision of the Equisetales, we may regard the genera _Calamites_, _Archaeocalamites_, _Equisetites_, _Equisetum_, _Phyllotheca_ and _Schizoneura_ as so many members of the Equisetales, without insisting on a classification which cannot be supported by satisfactory evidence. Our knowledge of _Calamites_ is fairly complete. Abundant and well-preserved material from the Coal-Measures of England, and from Permo-Carboniferous rocks of France, Germany and elsewhere, has enabled palaeobotanists to investigate the anatomical characters of both the vegetative and reproductive structures of this genus. We are in a position to give a detailed diagnosis of Calamitean stems, roots and strobili, and to determine the place of this type of plant in a system of classification. _Calamites_ not only illustrates the possibilities of palaeobotanical research, but it demonstrates the importance of fossil forms as foundations on which to construct the most rational classification of existing plants. The close alliance between _Calamites_ and the recent Equisetaceae has been clearly established, and certain characteristics of the former genus render necessary an extension and modification of the definition of the class to which both _Calamites_ and _Equisetites_ belong. The Calamites broaden our conception of the Equisetaceous alliance, and by their resemblance to other extinct Palaeozoic types they furnish us with important links towards a phylogenetic series, which the other members of the Equisetales do not supply.