This the oldest species, from Stephanian beds at Grand’ Croix, differs in no essential features from the other representatives of the genus. It is from a study of the leaves of this type that Bertrand and Renault have obtained most of the facts with regard to the anatomy of _Poroxylon_ foliage. In the median region of the fleshy leaf the bundles are characterised by a comparatively large amount of centripetal xylem accompanied by a considerable development of secondary centrifugal tracheids: the bundles are connected laterally by both centripetal and centrifugal xylem and thus at certain levels in the lamina the vascular tissue has the form of a continuous plate (fig. 464, A, B). The veins become independent on branching and near the edge of the lamina they consist only of primary elements. Secretory sacs of elongated form are scattered in the homogeneous mesophyll, and thick stereome-strands underlie the epidermis. The epidermal cells are rectangular and rows of stomata occur on both surfaces. B. =CORDAITEAE=. =Cordaites=. Unger. A preliminary statement with regard to nomenclature may serve to remove possible misconceptions in connexion with the application of the generic name _Cordaites_. It has been the general practice to apply this name to certain forms of linear leaves which are particularly abundant in Carboniferous and Permian strata in Europe and North America, and in recent years a few palaeobotanists have substituted _Cordaites_ for _Noeggerathiopsis_ as the more suitable designation for Permo-Carboniferous specimens abundant in the rocks of Gondwana Land. It has been customary to assign to _Cordaites_ certain reproductive shoots, seeds, and stems described under the generic names _Cordaianthus_, _Cordaicarpus_, _Cordaicladus_, _Cordaioxylon_, etc. Stems agreeing anatomically in their main features with those of recent Araucarineae have long been attributed to _Cordaites_, but a few years ago a new type of stem was discovered which, though almost identical with that of _Cordaites_, is distinguished by the character of the primary xylem. For this new type the name _Mesoxylon_[543] was proposed. Nothing is known as to the reproductive organs borne on _Mesoxylon_ stems, but the leaves are externally at least indistinguishable from those referred to _Cordaites_. It is therefore obvious that when we apply the name _Cordaites_ to leaves or other plant-organs, under that designation are undoubtedly included specimens belonging both to _Mesoxylon_ stems and to stems with the characters of _Cordaites_ (_Cordaioxylon_). Further research may enable us to subdivide _Cordaites_ into more precisely defined types distinguished by well-marked morphological characters, but at present the only course would seem to be to restrict the term _Mesoxylon_ to petrified stems exhibiting the features of that genus and to retain _Cordaites_ as a comprehensive designation in accordance with the general account of the genus given in the following pages.