OBSERVATIONS ON DIATOM ACE A3. 
197 
One species included in tlie local list— Achnanthes inflata (Grunow) 
—Mono gramma ventricosa (Ehr.)= Stauroneis inflata (Kutz.)—is a 
very rare form, and appears to have been found only once pre¬ 
viously in Great Britain. I first discovered it in September, 1882, 
on moss (. Hypnum riparium) growing in the River Lea, on a stone 
step by my own garden at Hertford. I found it again in July, 
1885, under exactly the same conditions, but recent searches for 
it have proved fruitless. Some of the specimens were identified by 
Mr. Button of Norwich, who mentions it in bis list of the Diato- 
macese of Norfolk as AcJinanthes ventricosa (Ehr.), and who informs 
me that be once found it in some moss-washings in that county, 
and that be was not aware of any other British habitat. It has 
been found plentifully in New Zealand. 
I met with another species of Achnanthes in October, 1883, 
which I then considered to be Achnanthes suhsessilis. On a more 
careful examination, however, it did not appear to coincide clearly 
with any described species, and it was afterwards thought by Mr. 
Button to be what be termed a “starved condition” of Achnanthes 
coarctata. It was growing, at the time I found it, on a large 
flower-pot containing a plant of Agapanthus umhellatus, in my 
garden at Hertford. I found it subsequently, in September, 1885, 
growing on the same flower-pot, where I believe it still continues 
to flourish. 
Another instance of the localisation of a particular form is 
afforded in the case of Denticula sinuata , a species mostly found in 
mountainous districts. I have repeatedly found it growing in a 
small pond near Pembridge Lane, but never elsewhere in the 
county. 
The distribution of the Hiatomaceee is much assisted by the 
swollen condition of our streams and rivers after heavy rain, and in 
many cases they contribute an appreciable proportion of the muddy 
colouring matter which the rivers contain. On the 8th of April, 
1886, I took from the River Lea at Hertford, for the purpose of 
microscopical examination, one pint of water. The water was 
at the time much discoloured in consequence of there having been 
a considerable rainfall on the previous day, amounting to nearly 
half an inch. On carefully examining the solid matter contained 
in this small quantity of water, I was surprised to find upwards of 
fifty species of Diatomacese, some of which were very numerously 
represented. I should state that these were not such as were 
simply floating on the river, hut that, after agitating the surface of 
the water, the bottle was filled by placing its neck several inches 
below the surface. This distribution is further facilitated by dry 
dust in summer, and by what are known as dust storms, in the 
solid matter of which Ehrenherg detected upwards of a hundred 
species. 
The usual mode of increase of the Diatomaceae is by the process 
of self-division, by which one frustule becomes two frustules. This 
is accomplished by the division of the cell-contents into two portions, 
each one of which commences forthwith the formation of a new 
