It is important to recognise the fact that leaves included under the generic name _Cordaites_ were in many cases not borne on stems or branches with the anatomical characters of _Cordaites_. Scott in his account of the genus _Mesoxylon_ says, ‘I feel no doubt that most of the British specimens of Cordaitean leaves really belong to _Mesoxylon_, which is a much commoner type of stem in the Coal Measure petrifications than that of _Cordaites_ itself[555].’ Some of the Cordaitean leaves were probably attached to stems of the _Poroxylon_ type[556] and it is not improbable that, as investigations are extended, additional genera of vegetative shoots will be discovered provided with leaves similar at least in external characters to those which it is customary to refer to _Cordaites_. In the present state of our knowledge we cannot make use of anatomical characters as criteria by which to distribute the foliage of the _Cordaites_ form among the genera _Cordaites_, _Mesoxylon_, and _Poroxylon_, using these names as designations of certain types of anatomical structure. The specimen reproduced in fig. 465 is in all probability a piece of a leaf of _Cordaites principalis_, but on anatomical grounds Miss Benson[557] has made it the type of a new species, _C. Felicis_, and more recently Scott[558] has brought forward evidence supporting the view that it is a leaf of _Mesoxylon_. As, therefore, neither impressions nor petrifications of Cordaitean leaves can in the great majority of cases be referred with confidence to their respective genera of stems, pending fuller information the only course would seem to be to use the name _Cordaites_ in a comprehensive sense indicating in special cases where evidence is available the more precise systematic position of the specimen. The classification of Cordaitean leaves proposed by Grand’Eury[559] is based partly on the form of the lamina and in part on the equality or inequality of the ‘veins.’ The actual veins, which are embedded in the fairly thick mesophyll, do not directly affect the superficial ribbing on the carbonised impression of the leaves and, as seen in fig. 465, the most prominent hypodermal strands of supporting tissue which would appear as the main veins or primary ribs on an impression do not correspond in position with the vascular bundles. Although in some cases the largest stereome-strands coincide with the veins, forming the upper and lower parts of I-shaped girders the centre of which is occupied by the veins, this is by no means always the case. Grand’Eury has drawn attention to the difference between the upper and lower surface of some carbonised leaves: in _C. crassifolius_ (fig. 468, D)[560] there are five to seven finer ribs between each pair of primary ribs on one face while the other shows ridges and grooves with a rib corresponding to each. Attention is called on a later page to the variable character of the ribbing even on different parts of the same lamina.