2 DR. GREGORY, ON SOME NEW SPECIES 



any work to which I had access, neither in Ehrenberg's Atlas 

 of 1838, in Kiitzing, noi' in Rabenhorst. Nor did any English 

 observer know it. But I now find that Ehrenberg had de- 

 scribed it as P. borealis ten or twelve years ago, although his 

 figure, which, if published, appeared in the Berlin Transactions 

 or the Berlin Monthly Reports, was entirely unknown to all 

 our authorities in this country, none of whom, more than my- 

 self, have been able to consult Ehrenberg's very numerous 

 papers on the Berlin Transactions, or Monthly Reports, except 

 as quoted by Kiitzing or Rabenhorst, neither of whom noticed 

 this species. I mention these facts, to explain how it was, 

 that a species long ago described, and I believe figured, by 

 Ehrenberg, was regarded by all our authorities as new when I 

 found it in the Mull earth two years ago. And now I find, in 

 Ehrenberg's Microgeologie, not only that the species is com- 

 mon, which, so far as concerns Britain, I had myself noticed, 

 but that it is one of two specified by him as occurring in every 

 latitude and in every quarter of the globe, more uniformly 

 than any others.* The fact, that it so long escaped notice in 

 this country is explained by its occurring always scattered, and 



* The two forms named by Ehrenberg are Pinnularia horealis 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 accord- 

 ance 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 exuviaj, 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 Diatomacefe, 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 spicute 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 Dfatomaceaj, &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 Diatomacege, 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. 



