24 
DoNKiN, on Marine Diatomacece. 
7. PI. Wansheckii, n. sp.— Valve pale straw-coloured^ 
slightly convex^ linear lanceolate^ acute, slightly sigmoid near 
the extremities. Median line gently sigmoid, not central, 
not marginal. Length from 0*0045" to 0"005"; breadth 
about 0*0006". Longitudinal and transverse striae probably 
50 or more in 0*001". 
This species bears merely a generic resemblance to PI. 
angustum^ n. sp., and cannot be confounded with it. It 
differs widely from PL rectum, n. sp., in its much longer, 
acute, and gracefriUy sigmoid valve; in its median line, 
which is much less curved, and never approaches close to the 
margin; in its strise, which are not so fine; and in its 
habitat. 
Hab. Pools left by the tide, where the water is strongly 
brackish, at the mouth of the Wansbeck. It is not a littoral 
form. I have never met with a single specimen on the 
beach, Avhere the other allied forms, described in this paper, 
are abundant. This fact alone, independent of structural 
differences, would prove it to be distinct from any of 
these. 
8. PI. minutum, n. sp. — Valve a very pale-brown colour, 
oblong, acute, exceedingly convex. Median line strongly 
sigmoid. Length about 0*0025"; breadth about 0*0005". 
Striae very fine ; transverse distinct, probably 55 in 0*001''; 
longitudinal very obscure, owing to great convexity of the 
valve. 
The median line in this minute species, the smallest of the 
genus I have seen, is not so marginal near the extremities as 
that of PI. rectum. It differs also in its size and in the out- 
line of the valves. 
Hab. Cresswell, abundant. Frequent in some other 
localities. 
9. PI. angustum, n. sp. — Valve dull purple, rather opaque, 
exceedingly convex, linear; extremities acute, and slightly 
apiculate. Median line marginal, except in the middle, for 
a short space on each side of the central nodule, where it 
crosses the valve, forming a prominent ridge or keel. Length 
from 0*005" to 0*0055"; breadth about 0*0006", narrowest in 
the middle. Striae obscure, longitudinal, visible a little on 
one side of the median line. 
In this curious form the median line overlaps the margin 
of the valve on either side, and prevents its being seen in 
this situation. The great convexity (and opacity of the dry) 
valve renders it almost impossible to resolve its striae; al- 
though I have seen the longitudinal near the concave side of 
the median line. It is evidently allied to PI. rectum, but 
