144 MEDULLOSEAE [CH. 



fibres either as separate groups or in contact with the canals 

 (fig. 435, C). In the same peripheral tissue occur scattered 

 collateral vascular bundles (fig. 435, D) identical with those of 

 Myeloxylon. The outer cortex of the stem is marked off from 

 the more homogeneous inner region by a fairly distinct Une 

 where there is some indication of periderm. The anatomical 

 features are clearly shown in fig. 436, a photograph from a section 

 in Dr Kidston's collection. At a is an imperfectly preserved 

 vascular bundle with a crescentic group of secondary xylem 

 which is probably a leaf -trace that has just emerged from the 

 secondary cyhnder. Eenault speaks of these more or less circular 

 strands as possibly connected with reproductive shoots, but it 

 is more probable that they are homologous with the strands in 

 the pericycle and inner cortex of Medullosa and represent leaf- 

 traces before division into smaller collateral strands. Renault 

 describes the stem as possessing seven vascular cyhnders in the 

 apical region and suggests branching of the main axis as the catise 

 of the increase in number: there is, however, no evidence to 

 support such correlation. The two steles seen in fig. 435, A, 

 become merged at a lower level into a single stele of sinuous form 

 (fig. 435, B). 



Beyond the facts furnished by the leaf-trace bundles in the 

 outer cortex and the occurrence of two large scars about 5 cm. 

 in breadth on a stem figured by Renault, we have no positive 

 information as to the form of the leaves or the structure of the 

 reproductive organs. There is httle doubt that the fronds were 

 large and compound like those of most species of Medullosa. 

 There is, however, some shght evidence that Alethopteris Grandini 

 Brongn. and seeds of the Pachytesta type (fig. 497) were borne 

 on Colpoxylon stems; this rests solely on the association in the 

 Loire coal-basin^ of Alethopteris fronds with stems presenting 

 structural resemblances to Colpoxylon aeduense. 



The striking resemblance between Colpoxylon and Medullosa 

 Leucharti has led certain authors^ to propose the substitution of 



1 Grand'Eury (08) B. p. 1242. 



2 Goeppert and Stenzel (81) p. 125; Weber and Sterzel (96) B. p. 79. Solms- 

 Laubaoh [(97) p. 196] draws attention to the resemblances between the leaf-scars 

 of Colpoxylon and Medullosa. 



