20 
DoNKiN_, on Marine Diatomacece. 
I have already remarked that want, of symmetry in the 
opposite margins of the valve is a well- developed character of 
the genus Toxonidea. This is well illustrated in the present 
species^ and is more easily understood by examining the valve 
itself (fig. 1^ PI. Ill), which presents an appearance different 
from that of any other known diatom. One margin,, which, 
as in the next form to be described, I shall term the dorsal, 
follows very closely the curve of the median line, and is 
gently arcuate through the greater portion of its extent ; but 
near to the extremities of the valve it curves gradually back- 
wards in the opposite direction. The ventral margin, on the 
other hand, bears no relation to the median line ; it is almost 
linear, but slightly convex until near the extremities, where, 
after approaching close to the median line, it curves strongly 
backwards to its junction with the dorsal margin. The ven- 
tral margin then is linear elliptical. The striation appears 
to be as fine as that of PI. angulatum, and the areolation of 
the Pleurosigmata, with a sufficient power and illumination, 
is very distinct."^ The striae, however, are much more easily 
resolved as transverse than as oblique ; owing, doubtless, to 
some structural peculiarity of the valve not found in the 
Pleurosigmata. ^ When viewed with a good ^ objective, and 
very oblique light from the mirror, the striae always appear 
transverse, like a finely marked Stauroneis, unless the light 
fall upon it at a particular angle, when they come out dis- 
tinctly in an oblique manner. 
these in O'OOl'', by taking PI. angulatum as a standard, by which I visually 
compared them. It is therefore probable that I may be, in some instances, 
a little wide of the truth. If so, the error was unavoidable, as the liighest 
powder I possess is a superior one-fifth objective; with this it is quite im- 
possible to count lines so fine as those with which the forms in question are 
marked. But this is a deficiency of little practical utility, as the microsco- 
pist must learn to measure the marking of minute objects by the eye rather 
than by the micrometer. 
* With one of Smith and Beck's instruments, their one-fifth objective of 
100° aperture, No. 1 eye-piece, five inches of the draw-tube, and the illu- 
mination afforded by their achromatic condenser, having the central portion 
of the illuminating pencil cut off by a central stop, the areolation becomes 
very distinct ; but much more so with No. 2 eye-piece and the same length 
of draw-tube, giving a power of 655 diameters. 
t On discovering this species, on the 28th of June last, I sent a specimen 
to Mr. Shadbolt, as a very remarkable Pleurosigma^ which I termed PI. 
arcuatum., not having then detected the smaller form which led to the for- 
mation of the present genus. After examining it carefully, he wrote to me 
as follows : " Your PL arcuatum is undoubtedly new, but I have some 
doubts about the genus ; the aspect and general appearance is very like a 
Fleurosigma ; but under a low power it looks like a Stauroneis, owing to a 
peculiarity in the internal part of the frustule. The areolation is very dis- 
tinct, and is exhibited with a one fifth without difiiculty when properly 
illuminated." 
