10 BARKAS, ON DIATOMACE®. 
examination necessary before a decision can be arrived at as 
to their newness. 
I shall be happy to exhibit the new and doubtful forms to 
those members of the club who are acquainted with marine 
diatomacez, and their opinions will be esteemed a favour. 
The Pleurosigmata are characterised by a more or less 
naviculoid form when seen on the side view, and linear lan- 
ceolate form seen on the front view. The front views of the 
Pleurosigmata, like the front views of the Toxonidee, are 
only seen when the frustules are alive and moving in water, 
on which occasions they frequently roll over in such a manner 
as to exhibit the front views of the frustules, but of the 
thousands of frustules of Pleurosigma and ‘Toxonidea which I 
have prepared and mounted I[ do not remember one in which 
the front view is exhibited. That is not the case with Don- 
kinia and Amphiprora, as by their peculiar conformation 
they as frequently when prepared and mounted dry present 
their front aspects as those of their sides. 
Of the genus Toxonidea, so named by its discoverer in 
consequence of the median line resembling a bow, there are 
only three known species, 7. Gregoriana, T. insignis, and T. un- 
dulata; the two former have been found on the Northumber- 
land coast, where they are very numerous, as may be seen by 
examining the cabinets of Dr. Donkin, Rey. Mr. Taylor, 
Mr. Atthey, or that in my possession ; all the microscopists 
named haye gathered them from the Northumberland shores 
in very great numbers. TJ. undulata has not been found in this 
neighbourhood, but was obtained by Mr. Norman, of Hull, 
from the stomachs of Ascidians got by fishermen off the coast 
of Hull. 
Pleurosigma and Toxonidea are somewhat similar in the 
flexure of their median lines, and yet more closely resemble 
each other in the naviculoid forms of their front views. 
The frustules of Donkinia are exceedingly abundant on all 
parts of the Northumberland coast; they are obtainable at 
the mouth of the Tyne, on the Long sands and Whitley sands, 
both near high-tide mark and at the low-water zone; with the 
exception of Navicula gregaria, Cocconeis excentrica, and 
Attheya decora, they are the most common local marine forms. 
They have flexed median lines similar to the Pleurosigmata, 
and recognised by their side aspects only ; their discoverer 
ranked them among the Pleurosigmata, from which genus 
they differ entirely in their front views, as they do also from 
Toxonidee, and more nearly resemble the Amphiprore, the 
only difference being the presence of ale in the Amphiprore 
and their absence in the Donkinian frustules. 
