In these the striation is strong and coarse ; but in the smaller it is not 
far removed in character from that of the N. rhyncocephala of Smith, so 
common in our fresh-water gatherings. 
In publishing these figures, and in the paper which they are designed to 
illustrate, my object has been to direct the attention of observers to the 
existence of a large number of forms, very few of which have hitherto 
been noticed or figured, although they are of very frequent occurrence. 
There is hardly a fresh-water gathering of a mixed character, in which 
many of them are not found. But the only figured species which are 
probably referable to this group are Navicula rJiyncocejphalat Pinnularia 
peregrina^ P. exigua, and possibly Navicula inflata. 
I would by no means be understood as asserting that all these forms 
belong certainly to one species ; but it is plain that most of them are 
very much inclined to vary in outline, and thus, apparently at least, to 
pass into each other, retaining all the time the peculiar character of 
striation, and the aspect which marks the group. I must here add, that 
even in the number of strise, and in one type of form, considerable varia- 
tions occur. As the forms become larger, the striae decrease in number, 
and become stronger and more conspicuous, so that no precise number can 
be fixed on, as typical of the species. I have myself counted in some of 
the smaller forms, 22 or 24 striaa in -001 of an inch, while in the larger, 
the number falls to 14, or even sometimes to 12. But this variation in 
the number of strise in -001 of an inch is by no means confined to these 
forms. It may be seen in many species, perhaps in all those which vary 
much in size. 
One more fact deserves to be noticed, as favourable to the idea that these 
forms, at least in great part, are referable to one species. I allude to the 
circumstance, that I have never found one of these types alone; but that 
several of them, in some cases nearly all, occur together. 
The localities which best exhibit this fact, are Duddingston Loch, 
Lochleven, and the Dhu Loch in Glenshira, the recent mud of which is 
rich in several of these types. A gathering from a brook near Oban con- 
tains nearly all of them, and another from Norfolk, which I owe to the 
kindness of Mr. Bleakley, also presents a large proportion of these forms. 
But, as I have already remarked, they occur, more or less abundantly, in 
almost every mixed fresh- water gathering. 
This leads me to observe, in conclusion, that all the forms of that series 
which appear to be identical with P. peregrina^ occur abundantly in pure 
fresh water ; whereas P. peregrina is described as a form of sea water, or 
brackish water. The forms here described, having moniliform strise, should 
be referred to the genus Navicula^ if that character be regarded as decisive ; 
of which doubts may be entertained. But as it appears, by the observa- 
tions of the Eev. Professor Smith, that he found the striae of undoubted 
P. peregrina to be, in some examples at least, moniliform, there can be 
little difficulty in regarding this form as one of the types of Navicula 
varians. At all events, the form or type alluded to, as I have found it 
in Duddingston Loch and Lochleven, is one which varies remarkably in 
outline and proportions. 
The figures now given must be considered merely as intended to bring 
under the notice of observers a very common, yet a very interesting, group 
of forms ; but not as, in any respect, exhausting the subject, or as esta- 
blishing with certainty the view which regards all these types of form as 
varieties of one species alone. That question cannot be decided without 
more extended investigation. But at least the figures will show that 
there is, in certain forms, a remarkable tendency to variation in outline ; 
and I have already found that this tendency exists in a considerable num- 
ber of species, to a greater or less extent. Farther observations will also 
determine how far this tendency to variation extends, and whether it 
affects, not only the outline, but the character or number, of the striae, the 
appearance of the nodule or of the median line, and the general aspect. 
