172 WALKER-ARNOTT, ON MARINE DIATOMS. 
different source: he had obtained from M. De Brébisson 
Sigmatella Brébissoni of Kiitzing, a truly fresh-water spe- 
cies; and on the supposition that this was the same, he 
applied the name Brébissoni to his species from Lewes, 
assigning it a fresh-water habitat; but De Brébisson’s (and 
as it was he who discovered it at Falaise, and sent it to 
Kiitzing, so his specimens form a correct index to both Syne- 
dra armoricana and Sigmatella Brébissoni of Kiitzing) proves 
to be a mere variety of Nitzschia sigmoidea; while Smith’s 
N. Brébissoni was actually quite unknown to De Brébisson 
or Kiitzing. That it isa truly brackish-water species is further 
proved by its having been found in such situations at Hull by 
Mr. Norman. I have the same, unquestionably, in a slide 
got from the drip from the walls at Chester; but it is very 
scarce there, and may have been carried inland by spray and 
storms, in the same way as Navicula pusilla, and even N. 
Jenneri, are sometimes conveyed to a distance from the sea, 
and found among mosses on trees with a corky or cracked 
bark. Nitzschia Tenia is said to be from “ brackish water :” 
I have it from several such localities; but I have collected 
it also from clay-pools in brick-yards, near the Botanic 
Garden of Glasgow, at an elevation of about 80 feet above 
the sea. On the other hand, I have found N. acicularis 
sometimes in brackish water; so that both are probably 
fresh-water species, and carried down by streams. 
Cyclotella punctata, Sm., is stated to inhabit fresh water 
at Wisbeach. In his slide are chiefly fresh-water species, 
and amongst them Synedra minutissima, for which Smith 
assigns a fresh-water locality only, but which I uniformly 
find within the influence of the tide. Swriredla ovata is here 
copious, but that Diatom is more abundant in brackish than 
in truly fresh water. The same species has occurred to me, 
but very rarely, in a preparation from Breydon, Norfolk 
(collected by the late Mr. Wigham), along with Cyclotella 
Kitzingiana, Pleurosigma strigilis, Amphora affinis, and some 
others not found in fresh water; there are, however, some 
fresh-water species mixed. This has been more recently 
and copiously found by Mr. Norman, of Hull, in the 
Market Weighton Canal, at the end near the sea, and which 
is occasionally supplied from the sea. Here, although the 
fresh-water species predominate, there are other forms also, 
as Synedra acicularis, &e., which indicate its brackish 
tendency. This species appears to me to be a genuine 
Coscinodiscus, very closely allied to C. subtilis, and probably 
with C. Normani, Grev., and some others, forming only one 
species, varying according to the saline nature of the water, 
and other incidental circumstances. It is the middle of the 
