Diatomacedus Sand of Glenshira. 
8^ 
possible, in all cases, to represent in drawing, characters 
which, in the forms, are perfectly satisfactory. 
Since writing the former paper, however, I have observed 
an additional mark of distinction, which has even led me to 
doubt whether the form under consideration be a Navicula at 
all ; for it frequently occurs in what I may call packs, like 
packs of cards, in which six, eight, or more are laid flat and 
close on each other. I have represented one of these in fig. 1.* 
This is a character which 1 have not observed in any Navicula^ 
although it is easy to imagine that some species of the genus 
may occur in such groups. From the fact of these packs 
being so frequent in a deposit like this, so long water-tossed, 
it may be inferred, that the forms composing them are very 
firmly attached together in the living state. I must leave to 
better authorities to decide whether this be a Navicula or not, 
merely observing that it is a well-marked and beautiful species. 
I believe N. rliomhica to be a marine form, having seen it, 
with other marine species, in a recent gathering from the 
coast near Tantallan, Haddingtonshire. There were also 
some fresh-water, or rather brackish-water forms, derived 
from the mouth of a small brook near the spot. If it be 
marine, this will be another point of distinction between it 
and N. rJtomboides. I have seen no trace of it in all the very 
numerous fresh-water gatherings I have studied, though 
N. rhomboides is one of the commonest forms (222. )t 
2. Navicula maxima, n. sp. This was also figured in my 
former paper, but I now give some additional figures of it, 
both because I have since found much finer specimens, and 
in order to show its usual varieties. 
Form linear, broad, usually a little incurved at the middle, 
with broadly acuminate apices, as in fig. 2. Also linear, 
narrow and long, without constriction, as in fig. 2*. Some of 
this variety are very long and narrow ; and there are also 
forms intermediate between 2 and 2*, as in fig. 2**. Length 
from 0*0035" to 0*0065". Median line strong, usually some- 
what bent towards the central nodule, at least in the broader 
variety. Striae transverse, parallel, reaching the median line ; 
fine and close, about 50 in O'OOl" in the broader, consider- 
ably finer in the narrower variety. Colour of the valve in 
balsam, clear straw yellow. The valve is thick and convex, 
so that, when not lying quite flat, the edges become black. 
It is a very striking form, and frequent in the coarser densities 
of the prepared sand. 
From the figure formerly given, some have supposed it to 
be identical with N firma fi, Sm. As that form was not 
t This is the number attached to the species in the list given in Part I. 
