Diatomaceous Sand of Glenshira. 
45 
15. Navicula didyma^ var. 7. To the four preceding forms 
I add one more, which I do not venture to erect into a new 
species. It has the form and size of a very frequent form of 
N. didyma^ but with the entire or costate striae of Nos. 11 and 
12. This character would lead us to make it a Pinnularia^ 
were it not that de Brebisson, and even Professor Smith him- 
self, who gives it as a character of Pinnularia^ have referred, 
in N, pandura and N. nitida^ costate forms to the genus 
Navicula. At least I am so informed as to N. nitida, for I 
have not seen Smith's description of it, nor an authentic speci- 
men named by him. De Biebisson's figure of iV^ pandura 
speaks for itself. 
I have figured the costate form, which, for these reasons, I 
refer for the present to N. didyma, in fig. 15. No detailed 
description of it is necessary, and I need only say here, that I 
frequently meet with it in the Glenshira sand, along with the 
other forms of this group, ^which I have figured, and that, 
besides the two common forms of N. didyma^ well figured 
by Smith, our deposit contains one, if not two other varieties 
which have moniliform striae, and which I refer also to N. di- 
dyma^ a species which, like N. elliptica, Kiitz. (ovalis, Sm.) 
and iV. elHptica, Sm. {Smithii, Breb.), appears to vary much 
both in outline and general aspect. 
16. One of these is represented in fig. 16. It is frequent 
in the deposit. I call it JV. didyma, d. 
It is evident that all these constricted forms belong to one 
group, but how they are to be classified it is not easy to say. 
The following questions naturally occur : — 1. Do the costate 
forms constitute one or more species ? 2. Are the monili- 
form types of this group to be referred to one or more species ? 
3. Is it possible that all these forms, whether moniliform or 
costate, belong to one and the same species ? and if so, how 
is that species to be defined? 
If we refer them all to one species, or even if the form, 
fig. 15, be referred to N. didyma, or figs. 11 and 12 to Navu 
cula, what becomes of Professor Smith's definition of Pinnu- 
laria, and how is that genus to be distinguished from 
Navicula ? I do not pretend here to answer these questions ; 
but I may state, that the form fig. 15 has every appearance of 
being a variety of iV. didyma (agreeing precisely, as it does, 
in form and size with the commonest small form of that 
species, which is very abundant in the deposit) ; and if that 
be so, then we have moniliform and costate striae in the same 
species. I may add that I have made observations on 
N. ellipticay Kiitz. {N. ovalis, Sm.), a common fresh-water 
form, which tend to show that it passes into N, didyma, 
