226 STRUCTURE OF HILDENBRANDTIA FLUVIATILIS. 
if this should be the species noted by Kiitzing,.“ Species Algarum”’ (p. 
695), and this be the form of fructification, differs altogether in the 
latter from Kützing's fructificating characteristics of the genus Hilden- 
brandtia. S 
The cells of the thalloid expansion generally are not more than 4.4 ;th 
of an inch in diameter, and the layer itself so thin, hard, and firmly 
attached to the stone, that it requires to be scraped off under a mag- 
nifying glass with an almost equally thin and sharp knife for micro- 
scopie observation. I examined this Alga in February and May of 
this year (1864) with the same results, but do not consider my exa- 
mination at all satisfactory, and therefore hope to resume it when I have 
more leisure than at the present time. 
I have mentioned that some of the cells of the columns of the sca- 
brous spots were empty, meaning that they thus presented the appear- 
ance of ovi-cells, from which the contents had escaped in the condition 
of zoospores. Again, the division of the upper cell in some columns 
into two or four daughter-cells is remarkable; but to state where the 
two elements of fructification, viz. sperm-cells and germ-cells, respec- 
tively come from is not, at present, in my power. So this is all for 
future observation to determine. The thalloid expansion is extremely 
thin; the cells of the layer throughout extremely minute ; and the diffi- 
culty of bringing a piece of the layer i» situ under a high microscopic 
power for good observation, on account of the opacity of the flint on 
which it grows, so great, that as yet I have by no means, as before 
stated, examined the structure of this Alga satisfactorily. 
On going to the rocks on the seaside for a portion of H. san- 
guinea for comparison, I found iż also growing on flints only. I have 
found neither species yet growing on the sandstone pebbles. Both, 
from the extreme thinness and delicacy of their structure respectively, 
appear to prefer the surface of flints, probably for its smoothness. 
I have not found H. fluviatilis in any other place in the neighbour- 
hood of that mentioned. 
Kützing, if I am right in identifying the species found at Tidwell 
with his Z. fluviatilis, makes it a variety of his H. rosea, a marine 
species, p. 694, op. cit.; but, as before observed, if it bears no con- 
ceptacles such as those by which he characterizes the genus Hilden- 
brandtia,—then it can hardly be considered a species of this genus. 
The colour fades somewhat by drying and keeping, as clearly seen ` 
by one of the specimens, which was taken last February. 
