The old generic terms _Artisia_ and _Sternbergia_ illustrate another source of error which can be avoided only by means of a knowledge of internal structure. The former name was proposed by Sternberg and the latter by Artis for precisely similar Carboniferous fossils, having the form of cylindrical bodies marked by numerous transverse annular ridges and grooves. These fossils are now known to be casts of the large discoid pith of the genus _Cordaites_, an extinct type of Palaeozoic Gymnosperms. _Calamites_ and _Tylodendron_ afford other instances of plants in which the supposed surface characters have been shown to be those of the pith-cast. The former genus is described at length in a later chapter, but the latter may be briefly referred to. A cast, apparently of a stem, from the Permian rocks of Russia was figured in 1870 under the name _Tylodendron_; the surface being characterised by spirally arranged lozenge-shaped projections, described as leaf-scars. Specimens were eventually discovered in which the supposed stem was shown to be a cast of the large pith of a plant possessing secondary wood very like that of the recent genus _Araucaria_. The projecting portions, instead of being leaf-cushions, were found to be the casts of depressions in the inner face of the wood where strands of vascular tissue bent outwards on their way to the leaves. If a cast is made of the comparatively large pith of _Araucaria imbricata_ the features of _Tylodendron_ are fairly closely reproduced[150].