The Bunter Sandstone in which _Pleuromeia_ is the sole representative of plant-life, at least in certain localities, is usually considered to be a desert formation. We may not be far wrong in accepting Fitting’s suggestion that in this isolated species we have a relic of the sparse vegetation which was able to exist where the presence of lakes added a touch of life to the deadness of the Triassic desert. _Pleuromeia_ is recorded by Fliche as a rare fossil in the Middle Trias of France in the neighbourhood of Lunéville[166]. Herbaceous fossil species of Lycopodiales. The history of our knowledge of fossil representatives of the Lycopodiales, as also of the Equisetales, affords a striking illustration of the danger of attempting to found a classification on such differences as are expressed by the terms herbaceous and arborescent in the sense in which they are usually employed. As we have seen[167], the presence of secondary wood in stems of the Palaeozoic plant now known as _Calamites_ led so competent a botanist as Adolphe Brongniart to recognise a distinct generic type _Calamodendron_, which he placed in the Gymnosperms, reserving the designation _Calamities_ for species in which no indication of secondary thickening had been found. Similarly, the genus _Sigillaria_ was regarded as a Gymnosperm because it was believed to be distinguished from _Lepidodendron_ by the power of forming secondary vascular tissues; the latter genus, originally thought to be always herbaceous, was classed with the Pteridophytes. At the time when this unnatural separation was made between stems with secondary wood and those in which no secondary wood was known to exist, botanists were not aware of the occurrence of any recent Pteridophyte which shared with the higher plants the power of secondary growth in thickness provided by means of a meristematic zone. It is true that the presence or absence of a cambium does not in practice always coincide with the division into herbaceous and arborescent plants: no one would speak of a Date-Palm as a herbaceous plant despite the absence of secondary wood.