[AB] "Fossile Flora," 1852, p. 92, Table xli.
[AD] Brongniart, "Vegeteaux Fossiles," Plate vi., Figs. 7 to 12.
[AE] "Proceedings of the Vienna Academy," 1881. _Hostinella_, of this author, is almost certainly _Psilophyton_, and his _Barrandiana_ seems to include _Arthrostigma_, and perhaps leafy branches of _Berwynia_. These curious plants should be re-examined.
It is not surprising that great difficulties have occurred in the determination of fossil Algæ. Enough, however, remains certain to prove that the old Cambrian and Silurian seas were tenanted with sea-weeds not very dissimilar from those of the present time. It is further probable that some of the graphitic, carbonaceous, and bituminous shales and limestones of the Silurian owe their carbonaceous matters to the decomposition of Algæ, though possibly some of it may have been derived from Graptolites and other corneous Zoöphytes. In any case, such microscopic examinations of these shales as I have made, have not produced any evidence of the existence of plants of higher grade, while those of the Erian and Carboniferous periods, similar to the naked eye, abound in such evidence. It is also to be observed that, on the surfaces of beds of sandstone in the Upper Cambrian, carbonaceous _débris_, which seems to be the remains of either aquatic or land plants, is locally not infrequent.
[Illustration: Fig. 14.--Silurian vegetation restored. _Protannularia_, _Berwynia_, _Nematophyton_, _Sphenophyllum_, _Arthrostigma_, _Psilophyton_.]
Referring to the land vegetation of the older rocks, it is difficult to picture its nature and appearance. We may imagine the shallow waters filled with aquatic or amphibious Rhizocarpean plants, vast meadows or brakes of the delicate _Psilophyton_ and the starry _Protannularia_ and some tall trees, perhaps looking like gigantic club-mosses, or possibly with broad, flabby leaves, mostly cellular in texture, and resembling Algæ transferred to the air. Imagination can, however, scarcely realise this strange and grotesque vegetation, which, though possibly copious and luxuriant, must have been simple and monotonous in aspect, and, though it must have produced spores and seeds and even fruits, these were probably all of the types seen in the modern acrogens and gymnosperms.