170 
What are Marine Diatoms? By G. A. Watxer-Arnort, 
LL.D. 
In the Preface to the late Professor Smith’s second volume 
of ‘British Diatomacee,’ at p. 25, we find—“I feel per- 
suaded that a marine species will not flourish under fluviatile 
influences, nor a fresh-water form long survive when trans- 
ferred to a marine habitat. Still further, I believe that 
certain species are far more special in their tastes, some 
selecting mountain-torrents, and others fixing their habitation 
in boggy pools or alpine lakes; some being exclusively 
littoral, and others found only in the deeper parts of the 
sea.” As a mere speculation, this statement is of little con- 
sequence, but Smith applies it to the determination of 
species, when other means fail; thereby employing it, not as 
a theory, but as an ascertained fact. 
In general, the result at which I have arrived corroborates 
the theory in many particulars, but by no means to the full 
extent. A truly fresh-water species may be carried down 
to brackish water, and survive there; but the circulation is 
more languid, and consequently the amount of silex deposited 
smaller, so that, although the distance of the striz may be 
much the same, they become more faint. If the same 
species be carried into pools still more of a marine character, 
it is either killed or grows; but, in the latter case, the 
circulation is very languid, the silex almost or wholly absent, 
and the formerly conspicuous striation is now extremely 
obscure. I have very great doubts if Fragilaria striatula be 
not, on that account, a degenerated form of F. capucina. 
In the same way, truly marine species, when in brackish 
water, have their silex less in quantity and striz less visible, 
and in fresh water are either killed or grow very languidly, 
and become scarcely siliceous. But there are Diatoms which 
are peculiar to brackish water, and are not found in a good 
state either in the open sea or in fresh water. Epithemia 
musculus may be cited as one of these, and unquestionably 
there are others, as Navicula amphisbena and Nitzschia dubia, 
which flourish as well in brackish water as in fresh water 
remote from the sea. 
As to Diatoms occurring in the deeper parts of the sea, that 
is no doubt true; but it does not appear to me that any 
species can increase much without access to more atmo- 
spheric air and solar ight than can be obtained there; and, 
consequently, that most of what are found in dredging have 
been conveyed there by the receding tide from our own, or 
