68 Roper, on the Diatomacece of the Thames. 
which meets with such success in the localities I have before 
alluded to, that is, to examine them in a living state ; but, as 
far as I can judge from iny own experience, this affords a most 
unsatisfactory result ; and after a careful examination of the 
mud deposited at different points in the Thames, any one 
might easily arrive at the conclusion that the varieties to be 
met with were comparatively few, and, except for the exami- 
nation of some of the larger species, not worth the time neces- 
sary for extended observation. 
Having, some months back, brought home a bettleful of 
the black mud from the extremity of the Isle of Dogs, taken 
about half-way between high and low water mark, and for 
several nights successively submitted it to a careful examina- 
tion, the only species of Diatomacece I met with were a Trice- 
ratium favus, and several specimens of Coscinodiscus radiatus 
and Surirella splendida, I had laid it aside for some time, 
when it occurred to me that the same course of proceeding 
which is necessary to bring out the siliceous frustules from 
guano might prove equally efficacious with this Thames mud. 
Acting on this idea, I boiled a portion of it for some time in 
hydrochloric and afterwards in strong nitric acid, until the 
whole was perfectly clean : and, on mounting it, the result far 
exceeded my expectations ; for though impossible to form 
an accurate conclusion, I should imagine that, excluding the 
coarse sand, nearly one-fourth of the finer part of the residuum 
was entirely composed of the siliceous valves of different 
species of Diatomacece ; and the prevalence of marine forms 
also proves that, at the distance of nearly forty miles from 
the mouth of the Thames, their distribution is very similar 
to that previously described by M. Ehrenberg in the Elbe. 
The only observations on this point of the inquiry, as 
regards British rivers, that I have met with, are notices of the 
species which occur in the Humber, and in a paper by Mr. T. 
F. Bergin,* read before the Microscopical Society of Dublin 
in 1842, who, from a careful examination of the deposits of 
the Liffey, after a perusal of Ehrenberg's paper on the Mud 
Banks in the Harbour of Wismar, was led to a different con- 
clusion ; and stated it as his opinion that a few species of 
Navicula, not comprising 1-lOOOth part of the mass, were the 
only organized forms that occurred in the mud deposited by 
that river. The cause of this he attributes to the fact of the 
source of the river being so short a distance from the sea, and, 
liaving its rise in the mountains of Wicklow, the rapidity 
* Micro^scopic Journal, vol ii., p. fi8. 
