216 PROCEEDINGS OF SOCIETIES. 
fect disc, and the rays of this star always coincide with the compart- 
ment last described. The inner plate also shows marks indicating 
the position of the marginal (improperly so- called) spines; and under 
a high power shows also faint impressions of the areolze of the onter 
Pp late, which I consider proof that the two plates were in actual con- 
tact. It is this inner plate that gives the veiled appearance to this 
and other diatoms, and I take the “veil” in all cases as a visual proof 
of the existence of the inner plate. Dr. Carpeuter says of He/topelta, 
that a minute granular structure may be shown to exist over the 
whole of the valve—‘ that the circular areolation exists in a deeper 
ayer of the siliceous lorica.” 
Now, 1 am certain that Dr. Carpenter was mistaken in this last 
remark, though, perhaps, not in what he saw. He had simply 
observed a valve with the inside toward the eye. I have repeatedly 
seen them in this position, and with the same effect. I have also 
found what I take to be the inner plate of an Ompha/opelta entire ; 
but the evidence of its connection with that genus is not quite 
complete. 
A few weeks since I found a broken specimen of Coscinodiscus ; 
the hexagonal areolz were large and distinct, and extending beyond 
the broken edges, just as described in the Heliopelta, was another 
part of the dise, which was simply granular, with a milky aspect. 
This is the inner plate of the valve of that genus. Since that I 
have found numerous examples of the same kind, and am now 
satisfied that they are quite common, and that others as well as 
myself must have seen them often before, without being aware of 
their nature. Like the corresponding plate of Heliopelta, this is 
composed of spherical granules of silex, but instead of being in 
close contact, they are distant, and joined or cemented together by 
a thin plate of silex, the arrangement and place of the particles 
being governed by that of the hexagons of the outer plate, one 
granule being placed against each hexagon. By careful adjustment 
of the focus of the instrument, with a power proportioned to the 
size of the areole, the granules can be seen in the centre of the 
hexagons; care must, however, be taken not to confound an optical 
effect with the appearance of the granules; each areole is a minute 
lens, and so refracts the light as to give a bright or dark dot as 
the focus is changed, and the granules themselves contribute to 
this effect. Practice, however, will enable one to distinguish these 
effects. : 
The species Eupodiscus, Argus, and Rogersi, present strong evi- 
dence of the inner plates; so, also, do some specimens of Isthmia 
nervosa, of EHpithemia, Achnanthes, aud Polymyaxus coronalis, I 
thiuk I have seen indications of them in several otber genera. In 
some of the Pinnularia and Navicula there are appearances which 
I can explain only on the supposition that the valve is composed 
of two plates, as suggested by Schleiden. Sufficient, ] think, has 
been proved to warrant the generalisation that the valve of the 
Diatomacese consists of at least two plates of silex, the inner one 
of a structure more or less differing from that of the outer, giving 
