176 Transactions of the Royal Microscopical Society. 
apexes of the frustules of this form have no constrictions as seen in 
A and B, and never, so far as my knowledge goes, a constriction so 
great as in A; and (2) although the striation is, in a general 
sense, the same as in N. rhomboides, anyone thoroughly familiar 
with both could never mistake the one for the other; and to show 
that the character of the beading is distinctively that of rhomboides, 
a portion has been carefully drawn at 8, A, Fig. 1. This was done 
with a magnification of 450 diams., and reduced; while the same 
nature and arrangement precisely of hemispheres or beads were 
demonstrated in each of the other three forms in Fig. 1. 
The point is, if “shape” be the characteristic on which the 
determination of “ genus and species” depends, then surely A, B, C, 
and D are each of them separate species! Moreover, at times it 
happens that the two ends of a given frustule are not absolutely 
like each other. In Plate CLXV., belonging to my paper published 
in the last Journal,* it will be seen that in Fig. C the right end of 
the frustule is not like the left one. This is strictly true to nature. 
A constriction is found at one end and not at the other ; does this 
make it, and similar forms, specifically different? It was stated in 
the discussion that the drawing of the large specimen which I had 
sent to the Society had not the “rhomboidal outline”; but if I 
had considered that “outline” of the slightest moment, it would 
have been an easy matter to find one that had. Do I rightly 
understand that such a specimen, if found and presented, would 
have to be considered a distinct species from the one drawn ; and 
which one of the speakers (Mr. Ingpen) declared “he should hardly 
eall a rhomboides at all”? If this be true, I can only say that 
there is, in what is now known as N. rhomboides, plenty of work 
for those who desire to engage in the labour of describing and 
naming species, 
But it appears to me that this canon for determining the 
generic and specifie differences of diatoms cannot hold good. It 
certainly must be rejected by the biologist. The differences be- 
tween A and D, Fig. 1, are not greater than those seen in many of 
the varieties of several species of fern ; nay, they certainly are not 
greater than the difference existing between a bulldog and a grey- 
hound, or between a “ pouter” and a rock-pigeon. And I think 
that we should therefore have no right to consider them specifically 
different, even 2f the intermediate links could not be found; which 
they certainly can. 
But whilst we have all this variation in form, there is one 
characteristic that is practically unchangeable; it is the character 
of the “striation.” It is perfectly true that, although it is the 
rule that the smallest frustules have the most delicate striation, it 
ig not (so far as my experience goes) invariably so. Small frustules 
* The present paper was sent to the Secretary in January, but could not be 
read until March, the Presidential Address intervening. 
