An increased precision in knowledge derived from anatomical investigation often tends to demonstrate the untrustworthiness of criteria based on external features previously employed with confidence. This inevitable though, from the point of view of the systematist, inconvenient result of intensive study is well illustrated by the recent discovery of the stems named by Scott and Maslen _Mesoxylon_[721]. A separation of _Cordaites_ from _Mesoxylon_, which no doubt extended far beyond the British area, is possible only if well-preserved petrified material is available. The leaves of _Mesoxylon_, so far as our imperfect knowledge of them enables us to express an opinion, are constructed on a plan almost identical with those of _Cordaites_ and, as already stated, it is almost certain that many of the impressions referred to _Cordaites_ were borne on _Mesoxylon_ stems. An additional source of confusion is supplied by the _Cordaites_-like leaves of _Poroxylon_. It is evident, therefore, that even within the limits of the Carboniferous and Permian formations the recognition of true _Cordaites_ leaves must often be attended with considerable risk of error. Apart from the possible confusion between the foliage of _Cordaites_ and _Mesoxylon_ there are other difficulties as regards detached leaves which depart more or less widely from the typical Cordaitean form. Leaves such as _C. circularis_ (fig. 468, B) and _C. grandifolius_ emphasise the lack of any thoroughly satisfactory dividing line separating single pinnules of _Cardiopteris_ or _Cyclopteris_ on the one hand and leaves of _Psygmophyllum_ on the other from _Cordaites_. The petrified buds described as _Dolerophyllum_[722] have been quoted by several authors as examples of unexpanded shoots of _Cordaites_ though anatomical evidence warrants a generic separation. In the case of species founded on leaves described in this chapter as _Cordaites_ it should be remembered that further research may necessitate an alteration in nomenclature.