DoNKiNj on Marine Diatomacea. 
scopical Society I have, by the kindness of Mr. Eoper, and 
of Dr. Montgomery, of Penzance, been favoured with slides of 
PI. rectum, n. sp., gathered at Penzance. The former gen- 
tleman informs me that Professor Walker- Arnott has very 
recently discovered it there, and named it Amphiprora Ralfsii. 
But as my description of this form has already been made 
public, I have retained it in the present contribution. I 
must also add, that however reluctant I may be to dissent 
from so high an authority as Professor Arnott, yet I am 
convinced that this species is a Pleurosigma, and not an Am- 
phiprora. My reasons for holding this opinion are as follows : 
1st. Because the S. V. has a sigmoid appearance; the sig- 
moidure resulting from the opposite margin, near each extre- 
mity, following the convex curve of the contiguous median 
line. 2dly. Because the structure of the valve is that of a 
Pleurosigma — the striae being distinctly longitudinal and 
transverse, — as may be observed without difficulty by using a 
good ^ objective and the achromatic condenser with a central 
stop, aided by careful manipulation. In this way the strise, 
though very fine, come out very sharp and distinct. The 
only evidence on which it appears to me the supposition rests 
of its being an Amphiprora is the contour of the F. V., which 
is keeled and constricted. But the discovery of PI. lanceo- 
latum and PI. carinatum show that this character, apart from 
the structure of the valve, cannot be relied upon. In both 
these species the T. V. is very distinctly keeled, and as deeply 
constricted in the middle as most of the Amphiprorce ; and 
yet their valvular structure proves them to be Pleurosigmata 
— the striae being very distinctly oblique, and indicating the 
hexagonal ar eolation of one section of that genus. 
These facts show that the outline of the frustule and of the 
S. V. is not sufficient to determine whether a particular 
species belongs to the one genus or to the other, and that in 
every instance the structure of the valve is the only character 
on which any reliance can be placed. The presence or ab- 
sence of lateral or marginal plates, which Professor Gregory 
has recently shown to be so universally developed in the 
Amphiprorce, is also a feature of great importance, and will 
materially assist in the discrimination of species. In con- 
cluding, I may add that all the new species of straight Pleu- 
rosigmata described in this paper are closely allied to the 
Amphiprorce, and form a connecting link between this latter 
genus and the one to which they belong. 
It is necessary here to add that, while these pages were 
passing through the press, I have been informed by Mr. Roper 
that PI. lanceolatum, n. sp., PI. Ill, fig. 4, is identical with 
