under the coal beds; while trunks of the same plants were the most abundant fossils of their partings and roofs. The growth of successive generations of Sigillariæ was, in fact, found to be the principal cause of the accumulation of a bed of coal. Two species form the central figures in our illustration. [Illustration: Fig. 13.--GROUP OF CARBONIFEROUS PLANTS, RESTORED FROM ACTUAL SPECIMENS. (_a_) Calamites (type of _C. Suckovii_). (_b_) Lepidofloios, or Ulodendron. (_c_) Sigillaria (type of _S. reniformis_). (_d_) (type of _S. elegans_). (_e_) Lepidodendron (type of _L. corrugatum_). (_f_) Megaphyton (type of _M. magnificum_). (_g_) Cordaites, or Pychnophyllum (type of _C. borassifolia_).] Along with the trees last mentioned, we observe others of a more graceful and branching form, the successors of those Lepidodendra already noticed in the Devonian, and which still abound in the Carboniferous, and attain to larger dimensions then their older relations, though they are certainly more abundant and characteristic in the lower portions of the carboniferous. Relatives, as already stated, of our modern club-mosses, now represented only by comparatively insignificant species, they constitute the culmination of that type, which thus had attained its acme very long ago, though it still continues to exist under depauperated forms. They all branched by bifurcation, sometimes into the most graceful and delicate sprays. They had narrow slender leaves, placed in close spirals on the branches. They bore their spores in scaly cones. Their roots were similar to Stigmaria in general appearance, though differing in details. In the coal period there were several generic forms of these plants, all attaining to the dimensions of trees. Like the Sigillariæ, they contributed to the materials of the coal; and one mode of this has recently attracted some attention. It is the accumulation of their spores and spore-cases already referred to in speaking of the Devonian, and which was in the Carboniferous so considerable as to constitute an important feature locally in some beds of coal. A similar modern accumulation of spore-cases of tree-ferns occurs in Tasmania; but both in the Modern and the Carboniferous, such beds are exceptional; though wherever spore-cases exist as a considerable constituent of coal, from their composition they give to it a highly bituminous character, an effect, however, which is equally produced by the hard scales supporting the spores, and by the outer epidermal tissues of plants when these predominate in the coal, more especially by the thick corky outer bark of Sigillaria. In short, the corky substance of bark and similar vegetable tissues, from its highly carbonaceous character, its indestructibility, and its difficult permeability by water carrying mineral matter in solution, is the best of all materials for the production of coal; and the microscope shows that of this the principal part of the coal is actually composed.