DoNKiN^ on Marine DiatomacecB. 
31 
This splendid species I found plentiful in gatherings from 
Cresswell and Linemouth. In a gathering from Newbiggin, 
in which the next form abounded_, I could not detect a single 
frustule. It occurs on several other parts of the coast. 
2. Tox. insignis, n. sp. — Valve straw-coloured ; dorsal 
margin strongly arcuate^ ventral linear; extremities sub- 
acute^ on dorsal margin produced. F. V. linear lanceolate^ 
only seen in the living frustule. Median line not central_, 
strongly arcuate near the centre. Length from 0*0048" to 
0-006" j breadth of S. V. about O'OOl". Strise very fine, pro- 
bably from 75 to 80 in 0-001". 
This very remarkable form in its outline,, in short speci- 
mens especially, very much resembles a strung bow or a 
cocked hat. The strongly arcuate curve of the dorsal 
margin ceases a short distance from the extremities of the 
valve j the margin then pursues an almost linear course to its 
termination, thus giving the extremities on this side a pro- 
duced appearance. The almost linear ventral margin at each 
extremity curves gently backwards. The median line is most 
gracefully arcuate; it curves strongly towards the ventral 
margin, and after nearly approaching it, continues an almost 
rectilinear course, though just perceptibly backwards to the 
terminal nodules, which are strongly curved to the dorsal 
side. The median line is far from central, being situated no 
great distance from the ventral margin. 
The strise are remarkably fine and most difiicult to ex- 
hibit, and, as in the preceding species, come out transversely 
much more easily than as obliquely. The areolation I have 
observed with a ^, but with this power it is very faintly seen, 
even with the most favorable illumination and careful mani- 
pulation. The valve is undoubtedly a far more difiicult test 
object for a i or a J objective than any of the Pleurosigmata 
at present employed for that purpose, with the exception of 
PI. fasciola and PI. obscurum, which, however require the aid 
of a superior ^ or for the full exhibition of the strise, 
owing to theiT faintness . 
I have frequently examined the living frustule of this 
species; it moves through the water with the S. V. upper- 
most, occasionally turning on its dorsal surface for a few 
seconds, thus exhibiting a good view of the T. V. 
Hab. Frequent at Cresswell, and near Newbiggin abun- 
dant. 
Pleurosigma. 
Ooservations made on some of the species of this genus, 
about to be described, have convinced me that sigmoid flexure 
