BARKAS, ON DIATOMACE. 9 
by Dr. Donkin, and enrolled by him among the Pleuro- 
sigmata, was finally constituted into a new genus, and named 
by Mr. Ralfs, Donkinia, in honour of its discoverer. 
All the four genera belong to the family Naviculee, in 
which family there are already nineteen genera. 
I propose to select this evening the following for observa- 
tion, Pleurosigma, Toxonidea, Donkinia, and Amphiprora, 
as they are nearly related, and bear in many respects a con- 
siderable resemblance to each other, in one or more of the 
aspects in which they may be viewed. 
They are all free forms, and are found on our own coasts ; 
some appear to be peculiar to the Northumbrian shores, as up 
to the present time they have either not been found at all 
in other parts of the kingdom, or found so rarely as to render it 
probable they were there asstray frustulesrather than that they 
were in their natural habitats. Evidence of this, however, 
is only negative, and negative evidence is always doubtful.* 
The mode of gathering diatoms from the open sea-shore 
adopted by Dr. Donkin, Rev. Mr. Taylor, Mr. Atthey, and 
others, has now been before the world for eight years, and 
yet nothing approaching the work done by the gentlemen 
just named has been accomplished in any other parts of the 
kingdom; even the indefatigable Mr. Norman, of Hull, has 
not found on the Hull coast any of the Toxonidez, or 
Amphiprore, only a few of the Pleurosigmata, and not a 
single frustule of Donkinia, the whole of which genera 
abound on the Northumberland coast, extending from the 
mouth of the Tyne to Alnmouth. 
Many of the Pleurosigmata, and some of the Amphiprorz 
and Donkinie are found nearly as far up the beach as high- 
tide mark, but Toxonidee I have only found near low-tide 
margin. The four genera are characterised by having flexed 
median lines, by central and terminal nodules, and by being 
more or less strongly striated, the striz in some being very 
coarse and broad, and in others so delicate and close as to be 
visible only by the highest powers and most careful illumina- 
tion. 
This evening I propose to bring before you ezghteen species 
of Pleurosigma ; eight of which are well-known, six of which 
are doubtful, and four I have good reason for believing are 
new and undescribed. With respect to the doubtful and the 
unknown it would be unbecoming to dogmatise, as some of 
the forms so closely approximate, and the differences which 
form species are so minute as to render the most careful 
* Since this paper was read, I have received slides of all the four genera 
from both the south and west coasts of England. 
