ABSTRACT
Having examined with some minuteness the fossil Diatoms in the Mull earth and in the Glenshira sand, both of which deposits yielded a very large number of species, I felt desirous to compare with them the species at present living in our waters. Accordingly I obtained, from various friends, gatherings from a great many different localities, both in England and in Scotland. Those which I have been able to study with some care, up to the present time, are almost all from fresh water, and, postponing to a future opportunity an account of the marine gatherings, I propose now very briefly to notice the results obtained from a number of fresh-water gatherings, more especially with reference to such species as are either altogether new or new to Britain.
The two forms named by Ehrenberg are Pinnularia borealis and Eunotia (Nitzschia, Sm.) amphioxys; Having lately examined about 60 small specimens of earth, found attached to plants in the University Herbarium here, and given to me by Professor Balfour, I find, in accordance with what is stated by Ehrenberg, that every one of these specimens of earth, which are chiefly from different parts of South America, contains diatomaceous exuviæ, and many of them in considerable quantity. I have detected, in examining only one slide of that part of each earth which is insoluble in acids, not only Diatomaceæ, to the extent of from 20 to 40 or even 50 species, in each case, most of which are identical with British forms, but also spiculæ of Sponges, and many Phitolitharia, exactly as Ehrenberg has done in the numerous similar earths analysed by him. It is most remarkable, that the two species above named occur in at least four-fifths of all the exotic earths I have yet examined; and one of them, P. borealis, in very nearly the whole of them. I may add, that I seldom explore a fresh-water gathering at home without finding one or both of these two species. Sufficient attention has not yet been paid to the fact of the invariable presence of Diatomaceæ, &c., in all earths in which plants are found. Ehrenberg, in his ‘Microgeologie,’ has established the fact as an universal one, and pointed out the important bearing it has on the growth of the soil. Indeed, it is difficult to imagine a more effectual agent in the transference of silica from the waters to the solid earth, than the growth of Diatomaceæ, the shells of which are as indestructible as their multiplication is rapid. Ehrenberg is of opinion that they live in the soil, as well as in water, and the constant presence of moisture in the soil renders this conceivable. Although the proportion of silicious matter, dissolved in ordinary water, is but small, it is evidently sufficient to supply the shells of millions of Diatoms in a very short time; and it is therefore probable, that as fast as it is extracted from the water by them, it is dissolved from the rocks or earths in contact with the water ; so that the supply never fails.
The species are numbered to correspond with the figures in Plate I.
I give a figure of Mastogloia Greυillei, first observed by Dr. Greville in a gathering from the Pentland Hills. I subsequently found it in my Lochleven gathering, but not having then seen Dr. Greville’s species, I did not at first recognise it. It is scarce on the gathering from Lochleven, but will probably be found in abundance in some part of the lake, or in some of the streams which supply it.
I am by no means sure that this form is correctly referred to the genus Cymbella. I at one time supposed it might be a Eunotia, or a Pinnularia, or possibly a Gomphonema. But the general opinion among those to whom I have shown it is that it comes nearest to Cymbella. It is marked, however, as doubtful. Some have conjectured it to be an abnormal state of some form, not specified. But it occurs in so many localities, always with the same characters, that 1 cannot but regard it as a normal and distinct species. Dr. Greville has recently met with it in various gatherings from the vicinity of the Bridge of Allan, and I have again found it in several from the neighbourhood of Hamilton.
Dr. Greville has also found it in a recent gathering from Duddingston Loch (April, 1855).
