22 
DoNKiN^ on Marine Diatomacece. 
of the valve is not so general as to render it applicable for 
one of the most important generic distinctions of a group so 
extensive as that of Pleurosigma. It appears to me^ that 
curvature of the median line, and not of the valve itself, must 
be looked upon as a characteristic feature of this genus,, and 
that all its known species prove these two important facts : 
Firstj that the median line may be sigmoid^ even strongly so, 
without any obvious curvature of the valve; for example, 
PI. lanceolatum and PI. carinatum (figs. 4 and 5, PL III). 
Secondly, that when the valve is sigmoid, it is so in confor- 
mity with the median line, as in PL hippocampus and others ; 
and that, although the whole valve may not observe the same 
amount of curve throughout as the median line, yet one 
margin at least, towards each extremity, generally does so. 
Section I. Striation oblique. 
3. PI. marinum, n. sp. — Valve straw-coloured, lanceolate, 
straight, slightly sigmoid near the extremities, obtuse. Median 
line sigmoid on each side of the central nodule. Length 
0-0055" to 0-006"; breadth of S. V. about O'OOl". Striae 
probably from 45 to 50 in O'OOl". 
The well-marked sigmoid flexure of the median line, on 
both sides of the central nodule, at once distinguishes this 
from any other British species belonging to the present sec- 
tion, and renders it easy of recognition. 
Hab. Newbiggin North Sands, plentiful. This is the only 
locality in which I have found this species. 
4. PI. lanceolatum, n. sp. — Valve straw-coloured, perfectly 
straight, broadly lanceolate, acute. Length from 0-0055" to 
0-006"; breadth from O'OOl" to 0-0014". Median line 
straight, or gently sigmoid in the middle; terminal nodules 
curved in opposite directions. Striae very fine, probably 
about 70 in O'OOl". 
This species is remarkable in consequence of the valve 
being free from the slightest sigmoid flexure. In most spe- 
cimens the only indication of curve exhibited by the median 
line is observable in the terminal nodules ; in others again, 
in addition to this, there is a very gentle curve in opposite 
directions on either side of the central nodule for a short dis- 
tance. The strise are remarkably fine, and require the most 
careful manipulation vdth very oblique light to render them 
visible with a superior ^ objective. The valve, therefore, is 
a test object of much greater delicacy than that of PI. angu- 
latum, though not equal to Tox. insignis.^ 
* As corroborative of mj opinion on this matter, I may adduce the tes- 
