Palaeobotanical literature contains numerous descriptions of Palaeozoic petrified wood occasionally enclosing an _Artisia_ pith-cast described under such names as _Dadoxylon_, _Cordaioxylon_, _Araucarioxylon_, etc., and regarded as portions of Cordaitean stems. It is, however, certain that much of this material belonged to stems other than those of _Cordaites_. Recent research has demonstrated the insufficiency of the secondary xylem alone, however well preserved, as a safe guide to generic position: stems identical in the structure of the secondary xylem differ in that of the primary portion of the stele, and it is on the characters of the latter tissues that several genera have recently been founded. _Mesoxylon_ affords a striking example of the importance of the primary xylem as a distinctive feature. As Gothan[646] points out, the species of _Calamopitys_ recently made the type of a new genus _Eristophyton_[647] would, in the absence of the primary xylem, probably be regarded as Cordaitean. It is important to recognise the limitations imposed by the imperfection of the material; we cannot in most cases determine whether a specimen should be referred to _Cordaites_ or _Mesoxylon_, and while it may be described as probably Cordaitean in affinity there remains the possibility that some of the Palaeozoic plants with secondary wood like that of _Cordaites_, if their reproductive organs were known, would not be included in the Cordaitales. Goeppert’s species _Araucarites Tchihatcheffi_, which Renault[648] quotes as _Cordaites_, has recently been assigned to a new genus _Mesopitys_[649] because of certain distinctive features of the primary xylem. Additional examples might be quoted pointing to the tendency of recent and more thorough investigation to establish the fact that the occurrence of Permo-Carboniferous wood of the Araucarian type does not necessarily denote the existence of _Cordaites_. The question of nomenclature is necessarily raised in this connexion.