Roper, on the Diatomacece of the Thames. 77 
might be referred to his Odontidium tahellaria : it is peculiar, 
from the strongly-marked cross strioiy which occur on each side 
of the valve ; the length is l-1385th and the breadth l-1750th 
of an inch. The form of the valve is similar to Ehrenberg's 
figure of Staurosira construens* which he describes "as a 
four-angled Fragilaria^ separated from the nearly allied genus 
of Amphitetras, by the absence of openings at the four angles,-[* 
but without authentic specimens for comparison, it is im- 
possible, from the small outline figure he gives, to refer it 
with certainty to this genus. Mr. West informs me he has 
met with it from many other localities.^ From the Thames 
near Gravesend I have lately obtained a large and fine spe- 
cimen of Coscinodiscus, about the l-107th of an inch in 
diameter. It has a smooth spot in the centre of the valve, and 
with that exception is covered with hexagonal cells, radiating 
towards the circumference. Mr. Smith informs me it is quite 
new to him, but approaches somewhat to C. marginatuSj but 
differs from the descriptions given of that species. I have at 
present no other forms, either from this locality or at Hammer- 
smith, that call for special notice. 
From the foregoing observations it appears that at the 
distance of at least fifty miles from the sea, the deposits of 
the Thames are still, to a certain extent, influenced by marine 
forms of life, and that at Greenwich, which is about forty 
miles from the mouth of the river, a most distinct marine 
character is shown by the examination of the species of 
Diatomacece which occur there. I think it very probable 
that many species are only brought up by the flood-tide, and 
being unable to exist in the slightly-brackish water, the 
siliceous skeletons are merely deposited in those parts of the 
river least subject to disturbing causes, and that they would 
rarely be met with in a living state. That they have a per- 
ceptible influence on the formation of shoals and mud-banks 
in the bed of the river there can be no doubt ; and the great 
abundance and general distribution of species serve to illus- 
trate the occurrence of similar deposits in a fossil state, at 
localities now far removed, by alterations in the earth's surface, 
from the streams or harbours in which they were originally 
deposited. 
Another point, probably worthy of attention, is the in- 
fluence these organisms have in the formation of deltas at the 
* See Berlin Academy Transactions, 1847, tab. 1, fig. 44. 
t See Berlin Academy Proceedings, 1843, p. 45. 
% From specimens I have lately seen of Odontidium Harrisonii, W. S.^ 
I am inclined to believe that this may be a small form of that species 
rather than 0. tahellaria. As it is a doubtful form I have not included it 
in the lists. 
