The Upper Laurentian and Huronian have as yet afforded no evidence of land vegetation. The Cambrian, as already stated, abounds in remains of sea-weeds; but though the forms which have been named _Eophyton_ have been regarded as land plants, this claim is, to say the least, very doubtful; and I have as yet seen nothing of this kind which did not appear to me to be merely markings made by objects drifted over the bottom or remains of marine plants. Yet in the Upper Cambrian there are wide surfaces of littoral sandstone often containing minute carbonised fragments, and which might be expected to afford indications of land vegetation, had such existed. I have myself devoted many days of fruitless labour to the examination of the large areas of Potsdam sandstone exposed in some parts of Canada. But as these rocks were evidently formed along the borders of a Laurentian continent capable of supporting vegetation, we may still hope for some discovery of this kind, more especially if we could find the point where some fresh-water stream ran into the Cambrian sea. [Illustration: FIG. 83.—_Protannularia Harknessii_ (Nicholson). A Siluro-Cambrian Plant, from the Skiddaw series.] The oldest plants, probably higher than Algæ, known to me by their external forms, are those described by Nicholson[25] from the Siluro-Cambrian Skiddaw slates of the north of England (Fig. 83). Their discoverer has named them _Buthotrephis Harknessii_ and _B. radiata_,[26] stating, however, that these two species are not improbably portions of the same plant, and that its form is rather that of a land plant than of an Alga. The specimens of these plants which I have seen appear to me to support the conclusion that they represent one species, and this allied to the _Annulariæ_ of the Devonian and Carboniferous periods, which probably grew in shallow water with only their upper parts in the air, and bore whorls or verticles of narrow leaves. They were either relatives of the Mare’s-tails, or of the Rhosocarps, of our modern swamps and ponds. [Illustration: FIG. 84.—American Lower Silurian Plants.—After Lesquereux. _a_, _Sphenophyllum primævum_. _b_, _Protostigma sigillarioides_.]