upper leaf-gap is closed and form part of an inner cylinder like the discontinuous ellipse formed by the two bands _c_. The section of a branch-stele is seen at _a_. [Illustration: Fig. 453. A. _Ptychoxylon Levyi_; transverse section of stem; _a_, stele of branch. (After Renault.) B. _Cycadoxylon Fremyi_; transverse section of stem. (After Renault.) C. _Cycadoxylon robustum_; centre of stem. 1–4, diagrams of sections of the stem of _Ptychoxylon_ at different levels. (Simplified from Renault.)] _Ptychoxylon_ differs considerably from _Medullosa_, which Renault included in the _Cycadoxyleae_, in the plan of the vascular system: there is nothing corresponding to the ‘partial pith’ or primary region which forms the central portion of the plate- and snake-rings in _Medullosa_. The double leaf-trace and the absence of the _Medullosa_ type of hypoderm are other distinguishing features. The paired leaf-bundles suggest comparison with _Lyginopteris_ among other genera and, as Scott[484] points out, the internal arcs of inversely orientated tissue which sometimes occur in the peripheral region of the pith of _Lyginopteris_ (fig. 405, C, _c_) behave like the internal bands of _Ptychoxylon_ in occasionally joining the main cylinder at a leaf-gap; but the differences outweigh the resemblances. As regards the general arrangement of the vascular tissue in two irregular concentric circles and their connexions with one another, but not in the structure of the xylem and phloem, there is a similarity between this genus and such a Fern as _Matonia_. In the varying patterns formed by the vascular system at different levels in the stem _Ptychoxylon_ resembles the Ferns _Polypodium quercifolium_ and _P. heracleum_[485]. =CALAMOPITYEAE.= =Calamopitys.= Unger. In 1856 Unger[486] described several fragmentary petrifactions from Thuringian strata of Upper Devonian age, the majority of which he referred to the Calamarieae and the Rhachiopterideae. In an earlier publication[487] he gave a list of species including two families, the Haplocalameae and the Calamoxyleae, assigned by him to the group Calamarieae: in the Haplocalameae he placed the new genera _Kalymma_, _Calamosyrix_, _Calamopteris_, and _Haplocalamus_. These were subsequently examined by Graf Solms-Laubach and identified as portions of petioles, for the most part belonging to unknown stems. In his second family, the Calamoxyleae, Unger included the single genus _Calamopitys_ represented by the type-species _C. Saturni_. The type-specimens have been thoroughly investigated by Solms-Laubach[488] who instituted the family-name Calamopityeae and recognised a close anatomical affinity between _Calamopitys_ and _Lyginopteris_, a conclusion which led to the incorporation of Unger’s genus in the Pteridosperms. Further data have been supplied by Zalessky[489] and, more recently, by Scott and Jeffrey[490] who have recognised _Calamopitys_ in Lower Carboniferous beds in Kentucky.