The primary xylem of _Rhetinangium_ agrees in its exarch structure with the Palaeozoic genera _Sutcliffia_, _Megaloxylon_ and _Stenomyelon_, also with _Lygodium_ and some other recent Ferns: the secondary wood is of the manoxylic type like that of _Lyginopteris_, _Heterangium_ and other genera. In the general structure of the stele _Rhetinangium_ agrees with _Sutcliffia_ and, except in the exarch structure of the primary xylem, with the steles of _Heterangium_ and _Medullosa anglica_[472]; but the structure and origin of the leaf-traces are characters which mark it off from _Sutcliffia_. The sclerenchymatous bands in the inner cortex of _Heterangium_ are unrepresented in _Rhetinangium_, and in the latter genus the abundance of secretory sacs and ducts is a characteristic feature, moreover in _Rhetinangium_, the leaf-trace consists of several groups of primary xylem-elements. Dr Gordon regards _Megaloxylon_ as the type which comes nearest to _Rhetinangium_; but the differences in the structure of the secondary wood and the marked contrast between the leaf-traces are too pronounced to justify a preference for _Megaloxylon_ over _Heterangium_ in the order of affinity. Gordon considers that the undivided leaf-trace of _Rhetinangium_ may represent a form transitional between the simple leaf-trace of _Lyginopteris_ and the much divided type in _Medullosa_. The external position of the protoxylem is a character to which too much weight may easily be attached: the difference in position between the protoxylem of _Rhetinangium_ and _Heterangium_ is in some examples of the latter genus hardly perceptible. Kubart[473] speaks of the stele of his species _Heterangium Sturi_ as being almost exarch. The inconstancy in the position of the protoxylem in the xylem of Osmundaceous stems and in the primary bundles of _Eristophyton_ and other Palaeozoic genera is worthy of consideration in this connexion. [Illustration: Fig. 451. _Stenomyelon tuedianum_ Kidst. Transverse section of stem. The black patches represent leaf-traces. (After Kidston; × 1½.)] =STENOMYELEAE.= =Stenomyelon.= Kidston. _Stenomyelon tuedianum_ Kidston. The specimens on which this monotypic genus is founded[474] were obtained from the Lower Carboniferous rocks (Calciferous Sandstone series) at Norham Bridge, Berwickshire, Scotland. They consist of petrified pieces of a flattened stem, a fragment of a rachis and portions of laminae: there is no evidence as to the nature of the reproductive organs. The original form of the stem is obscured by the destruction of a considerable part of the cortex and the consequent flattening of the whole with the production of wing-like extensions of the imperfectly preserved tissues enclosing the almost cylindrical stele (figs. 451, 452).