An important contribution to our knowledge of _Calamites_ was supplied by Petzholdt[593] in 1841. His main contention was the Equisetaceous character of this Palaeozoic genus. The external resemblance between Calamite casts and _Equisetum_ stems had long been recognised, but after Cotta’s account of the internal structure it was believed that the apparent relation between _Equisetum_ and _Calamites_ was not confirmed by the facts of anatomy. Petzholdt based his conclusions on certain partially preserved Permian stems from Plauenscher Grund, near Dresden. Although his account of the fossils is not accurate his general conclusions are correct. The specimens described by Petzholdt differ from the common Calamite casts in having some carbonised remnants of cortical and woody tissue. A transverse section of one of the Plauenscher Grund fossils is shown in fig. 70. The irregular black patches were described by Petzholdt as portions of cortical tissue, while he regarded the spaces as marking the position of canals like the vallecular canals in an _Equisetum_. Our more complete knowledge of the structure of a Calamite stem enables us to correlate the patches in which no tissue has been preserved with the broad medullary rays, which separated the wedge-shaped groups of xylem elements; the latter being more resistant were converted into a black coaly substance, while the cells of the medullary rays left little or no trace in the sandstone matrix. The thin black line, which forms the limit of the drawing in fig. 70, external to the carbonised wood, no doubt marks the limit of the cortex, and the appendage indicated in the lower part of the figure may possibly be an adventitious root. It is interesting to note that Unger[594] in 1844 expressed the opinion, which we now know to be correct, that the coaly mass in the specimens described by Petzholdt represented the wood, and that there was no proof of the existence of canals in the cortex as Petzholdt believed. [Illustration: FIG. 70. Transverse section of a Calamite stem, showing carbonised remnants of secondary wood. From a specimen (no. 40934), presented to the British Museum by Dr Petzholdt from Plauenscher Grund, Dresden. ½ nat. size.]