XXXII] STENOMYELOX 183 



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 Rhetin- 

 angium', 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 Heter- 

 angium 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 1 speaks of the stele of 

 his species Heterangium Sturi as being almost exarch. The incon- 

 stancy in the position of the protoxylem in the xylem of Osmun- 

 daceous stems and in the primary bundles of Eristophyton and other 

 Palaeozoic genera is worthy of consideration in this connexion. 



STENOMYELEAE. 

 Stenomyelon. Kidston. 



Stenomyelon tuedianum Kidston. The specimens on which 

 this monotypic genus is founded 2 were obtained from the Lower 



FIG. 451. Stenomyelon tuedianum Kidst. Transverse section of stem. The black 

 patches represent leaf -traces. (After Kidston ; x 1.) 



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 : 



1 Kubart (14). 2 Kidston and Gwynne-Vaughan (12). 



