Another group which claims our attention is that of the _Calamites_. These are tall, cylindrical, branchless stems, with whorls of branchlets, bearing needle-like leaves and spreading in stools from the base, so as to form dense thickets, like Southern cane-brakes (Fig. 46). They bear, in habit of growth and fructification, a close relation to our modern equisetums, or mare's-tails, but, as in other cases we have met with, are of gigantic size and comparatively complex structure. Their stems, in cross-section, show radiating bundles of fibres, like those of exogenous woods, yet the whole plan of structure presents some curious resemblances to the stems of their humble successors, the modern mare's-tails. It would seem, from the manner in which dense brakes of these _Calamites_ have been preserved in the coal-formation of Nova Scotia, that they spread over low and occasionally inundated flats, and formed fringes on the seaward sides of the great Sigillaria forests. In this way they no doubt contributed to prevent the invasion of the areas of coal accumulation by the muddy waters of inundations, and thus, though they may not have furnished much of the material of coal, they no doubt contributed to its purity. Many beautiful plants of the genera Asterophyllites and _Annularia_ are supposed to have been allied to the _Calamites_, or to have connected them with the _Rhizocarps_. The stems and fruit of these plants have strong points of resemblance to those of _Sphenophyllum_, and the leaves are broad, and not narrow and angular like those of the true _Calamites_ (Fig. 45). [Illustration: Fig. 42.--Erect _Sigillaria_, standing on a coal-seam (S. Joggins, Nova Scotia).] [Illustration: Fig. 43.--_Lepidodendron corrugatum_, Dawson, a tree characteristic of the Lower Carboniferous, A, Restoration. B, Leaf, natural size, C, Cone and branch, D, Branch and leaves, E. Various forms of leaf-areoles. F, _Sporangium_, I, L, M, Bark, with leaf-scars, N, Bark, with leaf-scars of old stem, O, Decorticated stem (_Knorria_).] [Illustration: Fig. 44.--_Lepidophloios Acadianus_, Dawson, a lepidodendroid tree of the coal-formation, A, Restoration. B, Portion of bark (two thirds natural size), C, Ligneous surface of the same, F, Cone (two thirds natural size). G, Leaf (natural, size), K, Portion of woody cylinder, showing outer and inner series of vessels magnified, L, Scalariform vessels (highly magnified), M, Various forms of leaf-scars and leaf-bases (natural size).] [Illustration: Fig. 45.--_Asterophyllites_, _Sphenophyllum_, and _Annularia_. A, _Asterophyllites trinerne_. A^1, Leaf enlarged, B, _Annularia sphenophylloides_. B^1, Leaf enlarged, C, _Sphenophyllum erosum_. C^1, Leaflet enlarged. C^2, Scalariform vessel of _Sphenophyllum_. D, _Pinnularia ramosissima_, probably a root.]