DoNKiN^ on Mm'ine Diatomacem. 
23 
Hah. Plentiful along the coast between the Coquet and the 
Wansbeck. Newbiggin North Sands^ abundant. 
5. P/. carinatum, n. sp. — Valve straight^ linear lanceolate, 
acute, very convex, colour dull purple. Length about 
0-0046" ; breadth about 0*0005". Strise fine, probably from 
55 to 60 in 0*001''. Median line strongly curved on either 
side of the central nodule, until it approaches close to the 
margin of the valve, in which direction it continues to its 
termination ; its marginal portion forming a prominent ridge 
or keel, which is much more prominent on one side of the 
central nodule than on the other. This peculiarity gives the 
F. Y. an unequally keeled appearance, as seen in fig. 5 b. 
The keeled appearance of the F. V. at first led me to sus- 
pect that the present form ought to be referred to the genus 
Amphiprora. But the strong sigmoid flexure of the median 
line, and the distinctly oblique striation, together with the 
absence of marginal plates,^ which Professor Gregory has 
shewn to be so generally present in the members of this 
genus, has convinced me that it is a genuine Pleurosigma. 
The striae are not easily resolved in the S. V., owing to its 
great convexity; they however come out very distinctly in 
the F. V. 
Hab. Newbiggin North Sands, abundant ; Linemouth 
and Cresswell, frequent. 
Section II. Strice longitudinal and transverse. 
6. PI. rectum, n. sp. — Valve pale straw-coloured, very con- 
vex, linear, narrowest in the middle, straight, extremities 
rounded on one margin, somewhat obtuse. Median line 
strongly sigmoid ; marginal for the terminal half of its extent 
on either side of the central nodule. Length from 0*0045" to 
0*005" ; breadth about 0*0006''. Longitudinal and transverse 
strise distinct, fine, probably 60 in 0*001". 
The S. V. appears sigmoid at the extremities ; but it is not 
so. This appearance is due to one margin, on opposite sides 
near each extremity, following the convex curve of the 
median line. 
Hab. Frequent. At Creswell, abundant. 
timony of Mr. Shadbolt, who, after examining one of my specimens witli 
a power much higher than I possess, sent me the following reply : " Your 
PL la7iceolatum has markings as you indicate (oblique), which are ensily 
resolvable under my one-twelfth, but with difficulty by the one-fifth. They 
are much more difficult than those of PI. angulatum." 
* " On the Marine Diatomaceous Forms of the Clyde," ' Trans. Royal 
Soc. Edinb., vol. xxi, part iv, p. 32. 
