In _Mesoxylon poroxyloides_ Scott and Maslen[719], the twin-bundles of the traces unite soon after reaching the pith as in _M. Lomaxi_, the secondary tracheids have only two rows, or sometimes a single row, of bordered pits and the tracheids are rather smaller than in _M. Lomaxi_ (20–40μ as compared with 30–60μ) and the medullary rays are shallower. There is a particularly broad zone of spiral and reticulate transitional tracheids at the inner edge of the wood as in _Cordaites_ and in _Dadoxylon Pedroi_ (fig. 476). The leaves of this species are believed to be represented by the type described by Dr Benson as _Cordaites Felicis_ (fig. 465)[720], but, as already suggested, it is very probable that many or possibly nearly all the leaves from British Coal Measures described as _Cordaites_ may belong to _Mesoxylon_. The chief interest of the genus _Mesoxylon_ is its close resemblance in certain characters to _Cordaites_ and _Poroxylon_: the presence of strands of centripetal xylem in the perimedullary region is an important feature in which _Mesoxylon_ differs from stems assigned (under the generic name _Dadoxylon_) to _Cordaites_. _Mesoxylon_ differs from _Poroxylon_ in having a discoid pith like that of _Cordaites_, but a more important difference is the absence in the leaf-trace xylem of _Mesoxylon_ of bordered pits of the Araucarian type, whereas in _Poroxylon_ Araucarian pits occur in both the centripetal and centrifugal tracheids. In _Poroxylon_ the secondary xylem is manoxylic; in _Mesoxylon_, as in _Cordaites_, it is pycnoxylic. =The range of Cordaites and a consideration of other imperfectly known genera.=