PROCEEDINGS OF SOCIETIES. 215 
ence of more than one plate of silex in the valve of some three or 
four species of diatoms. Mr. Shadbolt (‘Trans. Mic. Soe.,’ Ist 
series, vol. iil, p. 49) describes the valve of Arachnodiscus Jupont- 
cus as consisting of two layers. Mr. Ralfs (‘ Pritchard’s [nfusoria,’ 
4th ed., p. 839) says the valves of Actinoptychus undulatus “ fre- 
quently consist of two dissimilar plates, one having the usual 
character, the other being triradiate and minutely punctate, and 
which has been described as a new species by Mr. Roper, who first 
observed it detached from the true valve. He and others have since 
found the plates im situ.’ Dr. F. W. Lewis (‘ Notes on New and 
Rarer Species of Diatomacee,’ Phil., 1861, p. 6), describing Navi- 
cula marginata, speaks of ‘the outer siliceous plate.” Schleiden 
(‘ Pritchard,’ 4th ed., p. 41) speaks of ‘‘two leaves lying one over 
the other.” Mr. Brightwell says of the lorica of Triceratium, that 
“the valves are resolvable into several distinct layers of silex, 
dividing like the thin layers of tale.”’ (Pritchard, p. 49.) These 
are all the authorities I can find that intimate the existence of more 
than one plate of silex in the valve. 
Ehrenberg describes several species of diatoms as “veiled ’’—a 
most happy term as expressive of the appearance of those species 
to which it is applied. Neither Ehrenberg nor any other microsco- 
pist has offered any explanation of the cause of this appearance. 
Among the species thus distinguished are the four species of Helio- 
pelta, though the fact is not mentioned in any of the published 
descriptions, all of which are more or less imperfect. 
Some time ago I found a broken specimen of Heliopelta, which 
exhibited clearly portions of the valve with the normal characters 
of the genus, and, extending beyond the broken edges, portions of 
another and inner plate of an entirely different structure. A few 
months since, Mr. J. 8. Melvin gave me specimens of a diatom, as 
possibly a new species. On examination of these I found that he 
had obtained the inner plate of the valve of Heliopelta Leuwenhoekii 
entire and perfect. I have since found other specimens in my own 
collection. This plate under low or medium powers shows only 
exquisitely fine lines; but with a high power (4) it is resolved into 
minute spherical granules of silex, arranged in paralleled rows, 
radiating towards the margin of the disc, placed in contact with each 
other, and cemented together at their peripheries, the cement filling 
the interstices. There is a distinct line corresponding to the divisions 
of the compartments of the outer plate; a triangular blank at the 
junction of these lines with the margin, a conspicuous feature in the 
view of the perfect frustule; a star-shaped blank in the centre, the 
rays of the star being in number one half of that of the compart- 
ments of the disc. Heliopelta has the disc divided into six to twelve 
rays or compartments, one half of them having distinctly hexagonal 
areolz, the alternate half havimg an entirely different kind of mark, 
which has never been perfectly described or figured. Dr. Carpenter’s 
description is, perhaps, the best, but his figure is one of the most 
inaccurate. (‘Carpenter on the Microscope,’ Phil., p. 290.) The 
blank star of the inner plate is also a conspicuous feature of the per- 
VOL. I1I,.—NEW SER. Q 
