and unquiet time, and, except in certain bands of iron-ore and some dark slates colored with carbonaceous matter, we find in it no evidence of vegetation. In the Cambrian a great subsidence of our continents began, which went on, though with local intermissions and reversals, all through the Siluro-Cambrian or Ordovician time. These times were, for this reason, remarkable for the great abundance and increase of marine animals rather than of land-plants. Still, there are some traces of land vegetation.
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[1] 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. G. 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. 
Fig. 1—Protannularia Harknessii (Nicholson) a probable Rhizocarp of the Ordovician period. 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, 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.[2] 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.
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
