There is a close similarity between the vascular systems of _Cladoxylon_ and _Medullosa_, but an obvious difference is the substitution of the oval, transversely elongated, pits on the xylem elements for the multiseriate pitting of _Medullosa_. In _Cladoxylon Kidstoni_ the pitting shows transitional forms between a narrow scalariform uniseriate type and a biseriate or triseriate arrangement similar to that in the Araucarineae and Cordaitales. In _Cladoxylon_, as limited by Unger, the presence of secondary wood is a generic feature, but by the inclusion of _Arctopodium_ and other forms this character no longer holds good. The inclusion of these more fern-like stems without secondary xylem brings _Cladoxylon_ (in the wider sense) into closer contact with _Asterochlaena_, a comparison previously suggested by more than one author. In _Medullosa_ the development of secondary xylem is on a larger scale than in _Cladoxylon_, and the vascular system of the former genus assumes a more complex form. Moreover the _Myeloxylon_ type of petiole, which is a distinctive feature of _Medullosa_, differs widely from any form of leaf-trace associated with _Cladoxylon_. =Völkelia.= Solms-Laubach. _Völkelia refracta_ (Goeppert). The generic name _Völkelia_[525] was proposed by Solms-Laubach[526] as a substitute for _Sphenopteris_[527] in the case of some petrified stems or petioles associated with fragmentary impressions of fronds from Lower Carboniferous rocks in Silesia. Both leaf-impressions and petrifactions were included in the genus _Sphenopteris_: Solms, while retaining Goeppert’s designation for the leaf fragments, proposed a new generic name for the petrifactions on the ground that there is insufficient evidence of their connexion with the leaves. The short account of Goeppert’s petrified specimen given by Graf Solms-Laubach in his ‘Fossil Botany[528]’ is supplemented by a fuller description in a later paper. The fragments of highly compound fronds are characterised by very small filiform ultimate segments, but the specimens are too imperfect to afford a clear idea of the habit of the leaf. The ‘stem’ bears a close superficial resemblance to that described by Unger as _Cladoxylon dubium_ (fig. 459, C, D) and was regarded by him as an example of that species: it contains several radially placed steles represented by fairly well-preserved xylem, but no phloem has been recognised. The steles vary in size and shape: five reach almost to the centre (fig. 461, A) and smaller xylem groups occupy a peripheral position. Each stele is excentric in structure and consists of (i) an outer zone of secondary tracheids of horse-shoe form in transverse section, but the apparent gap in the secondary xylem on the outer edge of each stele is due to the crushing of the tracheal tissue and to its smaller breadth in the distal part of each group; this is shown in fig. 461, B, where the apparent gap is seen to be occupied by distorted and crushed tracheids, _a_, identical with