The oldest plants known to me, and likely to have been of higher grade than Algæ, are specimens kindly presented to me by Dr. Alleyne Nicholson, of Aberdeen, and which he had named _Buthotrephis Harknessii_[H] and _B. radiata_. They are from the Skiddaw rocks of Cumberland. On examining these specimens, and others subsequently collected in the same locality by Dr. Gr. M. Dawson, while convinced by their form and carbonaceous character that they are really plants, I am inclined to refer them not to Algæ, but probably to Rhizocarps. They consist of slender branching stems, with whorls of elongate and pointed leaves, resembling the genus _Annularia_ of the coal formation. I am inclined to believe that both of Nicholson's species are parts of one plant, and for this I have proposed the generic name _Protannularia_ (Fig. 1). Somewhat higher in the Siluro-Cambrian, in the Cincinnati group of America, Lesquereux has found some minute radiated leaves, referred by him to the genus _Sphenophyllum_,[I] which is also allied to Rhizocarps. Still more remarkable is the discovery in the same beds of a stem with rhombic areoles or leaf-bases, to which the name _Protostigma_ has been given.[J] If a plant, this may have been allied to the club-mosses. This seems to be all that we at present know of land-vegetation in the Siluro-Cambrian. So far as the remains go, they indicate the presence of the families of Rhizocarps and of Lycopods.
[J] _Protostigma sigillarioides_, Lesquereux.
[Illustration: Fig. 1.--_Protannularia Harknessii_ (Nicholson), a probable Rhizocarp of the Ordovician period.]
If we ascend into the Upper Silurian, or Silurian proper, the evidences of land vegetation somewhat increase. In 1859 I described, in "The Journal of the Geological Society" of London, a remarkable tree from the Lower Erian of Gaspé, under the name _Prototaxites_, but for which I now prefer the name _Nematophyton_. When in London, in 1870, I obtained permission to examine certain specimens of spore-cases or seeds from the Upper Ludlow (Silurian) formation of England, and which had been described by Sir Joseph Hooker under the name _Pachytheca_. In the same slabs with these I found fragments of fossil wood identical with those of the Gaspé plant. Still later I recognised similar fragments associated also with _Pachytheca_ in the Silurian of Cape Bon Ami, New Brunswick. Lastly, Dr. Hicks has discovered similar wood, and also similar fruits, in the Denbighshire grits, at the base of the Silurian.[K]