The most striking characteristics of _Megaloxylon_ are: (i) the structure of the primary xylem, particularly the unusual form of the majority of the metaxylem tracheids, a form obviously correlated with storage rather than with conduction of water; (ii) the gradual spreading of the leaf-traces and their absorption as they descend into the main mass of the xylem; (iii) the exarch structure of the primary xylem. Confining our attention to the primary region of the stele; a comparison is at once suggested with _Heterangium_. In _Megaloxylon_ the peculiarities are the substitution of the large storage-tracheids for the normal xylem-elements; the greater irregularity in the groups of metaxylem; and an exarch instead of a mesarch structure. In these last features the primary xylem agrees with that of recent species of the Schizaeaceous Fern _Lygodium_ and the external protoxylem is a character shared with _Rhetinangium_. The occurrence of short tracheids similar to those of _Megaloxylon_ in the inner portion of the stele of the Osmundaceous Fern _Zalesskya gracilis_ (Eich.)[468] may be quoted as an example of parallel modification but, as Scott[469] points out, the resemblance has no phylogenetic significance. The secondary xylem though less parenchymatous than in recent Cycads agrees more closely with the manoxylic than with the pycnoxylic type. _Megaloxylon_ affords an interesting example of a combination of primary stelar anatomical features, comparable in the exarch position of the protoxylem with the stele of _Lygodium_, and secondary wood similar to that of _Lyginopteris_ and _Heterangium_. The large metaxylem tracheids may be regarded as derivatives of elements which in some ancestral type were structurally fitted for the rôle of water-transport and made up the xylem of a _Lygodium_-like stele with little or no secondary xylem. As the cambial activity increased and centrifugal xylem became a prominent feature, usurping the function of the centripetal xylem, the latter became modified and fitted for a new service. =RHETINANGIEAE.= =Rhetinangium.= Gordon. _Rhetinangium Arberi_ Gordon.