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 little doubt that the fronds were large and compound like those of most species of _Medullosa_. There is, however, some slight 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[404] of _Alethopteris_ fronds with stems presenting structural resemblances to _Colpoxylon aeduense_. The striking resemblance between _Colpoxylon_ and _Medullosa Leuckarti_ has led certain authors[405] to propose the substitution of _Medullosa_ for _Colpoxylon_. The resemblances though close are hardly sufficient to warrant this course. In _Colpoxylon_ the stelar system is simpler; there is no central region with star- or plate-rings as in _Medullosa Leuckarti_ but, as in _Medullosa anglica_, the vascular tissue consists only of large steles without a medullary system. _Colpoxylon_ differs from _M. anglica_ in the reduction in some parts of the stem of the vascular system to a single stele and, moreover, the primary portion of the steles is much more parenchymatous in structure and contains more irregularly anastomosing tracheal strands than is the case in _M. anglica_. The alteration in the pattern formed by the vascular system at different levels in some Medullosan stems, especially in _Colpoxylon_, may be compared with the varying disposition of the vascular strands in the thick dorsiventral rhizomes of _Polypodium heracleum_ Kunz. and _P. quercifolium_ L. In the rhizome of _P. heracleum_ there are two vascular systems, an outer, cortical, system in the form of a hollow cylinder composed of a lattice-work with polygonal meshes from which branches are given off to the roots, and a more complex medullary system that is concerned with the emission of leaf-traces. As shown by a series of drawings reproduced in an account by Klein[406] of the anatomy of these species of _Polypodium_, the inner system of steles consists of two cylinders connected towards the upper surface of the stem by a rounded arch of vascular strands; nearer the leaf-base the two cylinders meet and eventually a larger cylinder is produced partly from the upper halves of the two cylinders of the previous section and in part from the connecting arch: the remains of the two smaller cylinders become connected with the outer vascular system. These and other changes suggest comparison with _Colpoxylon_ as also with the stelar changes in the stem of _Ptychoxylon_. The comparison cannot be carried beyond the grosser features and is chiefly interesting as affording a further illustration of a similarity in plan between some recent Ferns and extinct Pteridosperms and other Palaeozoic genera.