This generic term[477] is applied to a few types of Permian and Upper Carboniferous stems possessing a vascular cylinder, which may reach a considerable breadth, of secondary centrifugally developed xylem and phloem enclosing a large pith containing either a narrow, peripherally placed, and more or less continuous cylinder of inversely orientated conducting tissue or scattered bands of centripetal xylem and phloem. The secondary xylem is manoxylic, while the internal vascular tissue recalls that of _Ptychoxylon_ and to a less extent the inverted arcs that are rarely met with in _Lyginopteris_ stems. A brief diagnosis of two species may serve to illustrate the genus: a third species is included in _Cycadoxylon_, but it is founded on material too incomplete to admit of satisfactory diagnosis. _Cycadoxylon Fremyi_ Renault. This Permian species[478] is represented by a piece of stem 2–2·5 cm. in diameter (fig. 453, B) characterised by (i) a fairly broad parenchymatous cortex with secretory canals and several hypodermal nests of sclerous tissue, (ii) a cylinder of secondary xylem and phloem nearly the whole of which is centrifugal, (iii) a large pith containing several scattered narrower bands or arcs of centripetally developed xylem and phloem. The tracheids, with 4–6 series of hexagonal pits, form radially disposed rows, 1–4 elements broad, separated by broad and deep medullary rays. Renault does not mention the occurrence of any primary xylem as distinct from the secondary centrifugal xylem, but in a section which I examined some years ago in his laboratory there appeared to be a group of primary tracheids. There are no anastomoses between the main cylinder and the internal bands of inversely orientated tissue. _Cycadoxylon robustum_ (Seward). This species[479] is based on a piece of stem from the Lower Coal Measures of Lancashire first described by Williamson and identified as an unusually large example of _Lyginopteris_. Williamson and Scott, while recognising certain features in addition to the large size of the stem, which must have reached 14 cm. in diameter, expressed the view that ‘there is a presumption that it really belonged to a _Lyginodendron_, or to some plant of the same type of structure.’ The examination of additional material led me to adopt the name _Lyginodendron robustum_, though I suggested that possibly _Cycadoxylon_ might be the more appropriate genus. Subsequently Scott[480] proposed the substitution of _Cycadoxylon_ for _Lyginodendron_.