WALKER-ARNOTT, ON ARACHNOIDISCUS, ETC. 199 
believe that Smith’s views are now almost universally 
adopted; at the same time there are some cases which re- 
quire careful consideration. 
Pleurosigma is said to have sigmoid valves, that is, flat, but 
bent laterally in the same plane, at the one end, in a differ- 
ent direction from what they are at the other. In Navicula 
the opposite character is not given, but is implied. In XN. 
Jenneri and N. convexa the valves, at first sight, appear to be 
sigmoid; but the cause of this is easily understood by 
examining the F. V.; from it, it will be seen that the eutire 
frustule is not simply bent to the right or left, but has a 
slight spiral twist ; so that when the valves are separated they 
do not lie flat, and the result is the apparent sigmoid median 
line: had the valve not been twisted, the median line would 
have been perfectly straight and central. In Pleurosigma 
I have seen no instance in which the living frustule is 
twisted ;* “the F. V. is either of a linear, or linear-lan- 
ceolate, form” (Sm.), and the 8. V. is sigmoid, with the 
median line nearly equidistant from the two sides; but after 
the valves are detached from the connecting zone they often 
become slightly twisted, and as they cannot then present a 
flat surface to the eye, the median line appears to approach 
nearer to the one margin than to the other. How far the 
amount of this mequality can be relied on for the distinction 
of species is doubtful; P. decorum is principally separated by 
Smith from P. formosum by such an appearance. I am not 
aware that this twist has been seen except in those species 
which have oblique striz, and it may be dependent on that 
structure. 
In Amphiprora the median line in the entire frustule is 
usually straight, and when not so this arises from the torsion 
of the frustule; in A. alata and A. paludosa the frustule 
presents both appearances, as shown at fig. 124 6, and 0’, 
and fig. 260 6, and 0’, of ‘Smith’s Diat.2. I have not, how- 
ever, myself detected the twisted form in these while the 
diatom was alive, and it is often not so even after being 
dried, or maceration in weak acid. When the valves are 
separated from the connecting zone, their tendency to assume 
the spiral form is much more striking, as exhibited by Smith 
* T have not myself the F.V. observed of the living frustule of any 
of the anomalous or distorted states which form the genus Zoxonidea of 
Donkin ; of these states I have seen five, one I would refer to Pleus. estuarii, 
a second, if distinct, to P. danceolatum, a third to P. transversale, a fourth 
to P. angulatum, and the fifth to P. strigosum. The third of these I have 
detected lately, but of it only one frustule; the others I have already 
noticed (p. 165). 
