to which I have already referred, several species are actually described as having been well ascertained; and when we read of two plants in particular, whose characters are so strong and so decided as Lichen digitatus and Lichen rangiferinus, we cannot suppose it possible that he or any other observer could have been deceived. The execution of the plates which accompany his paper has unfortunately not rendered justice to the apparent accuracy of his observations.
It will be seen on inspection either of the specimens or of the drawings, that probably all the plants belong to the Cryptogamia class, and are limited to certain species of them. The explanation of this is sufficiently obvious. It is evident that the siliceous depositions which contain these remains must have been formed in caverns and clefts of rocks, situations only occupied by a few species of this class of plants, and these chiefly Byssi, Confervæ, Jungermanniæ, and the Mosses most commonly so called, plants which require very little light. All the specimens which I have figured will accordingly be found to belong to one or other of these families, with the exception of a few which appear to be fragments of plants, and which I have been unable from their mutilated state to compare with any known species.
It may be asked whether the plants thus preserved are specimens of existing species, or whether they are, like those found entangled in the secondary strata, the remains of a former set of organized beings. The limited number of the specimens which I possess, and the obscurity which attends not only these remains but the present living species, prevents me from attempting an answer to this question on botanical evidence, and on my own knowledge. But the observations of Daubenton appear sufficient to decide that in some instances at least they consist of existing species. Since too the formation of