70 Transactions of the Royal Microscopical Society. 
ground for maintaining, as I think we are quite justified in doing, 
that the idiosyncrasy inherent in the “ Sporangial frustule,” 
combined with the varying material conditions attendant on vicis- 
situdes of climate to which the sporangial frustule is pre-eminently 
subject, is a main cause of difference of size in the parent frustules 
of these organisms ? 
Now with reference to the first question. It is a singular fact 
that although the connecting zone constitutes a very important 
portion of the frustular structure, it has by no means received the 
careful attention to which its characters and uses entitle it in any 
really natural classification of the Diatomacex ; and nothing pro- 
bably could show more clearly, therefore, how signally these 
characters and uses must have been misunderstood. 
In ‘The Microscope and its Revelations’ (p. 304) Dr. Car- 
penter thus describes the Diatomacex :—“ Like the Desmidiacex, 
they are simple cells having a firm external coating within which 
is included a mass of endochrome whose superficial layer seems to 
be consolidated into a sort of primordial utricle. ‘The eaternal coat 
is consolidated by silex, the presence of which in this situation is 
one of the most distinctive characters of the group. It is a mis- 
take to suppose that the casing is composed of silex alone. For 
a membrane bearing all the markings of the siliceous envelope 
has been found by Professor Bailey to remain after the removal of 
the silex by hydrofluoric acid; and although the membrane seems 
to have been presumed by him, as also by Professor W. Smith, to 
lie beneath the siliceous envelope and to secrete this on its suriace 
like a kind of epidermis, yet the author (Dr. Carpenter) agrees with 
the authors of the ‘ Micrographic Dictionary,’ in considering it 
much more likely that it ¢s the proper cellulose wall interpenetrated 
by sclea.” 
Again :—“ The impermeability of the siliceous envelope renders 
necessary” (Dr. Carpenter says) “some special aperture through 
which the surrounding, water may come into relation with the 
contents of the cell. Such apertures are found along the whole of 
the line of suture in disk-like frustules; but when the diatom is 
of an elongated form, they are found in the extremities only. 
They do not appear to be absolute perforations im the envelope, 
but are merely points at which the siliceous impregnation is want- 
ing, and these are usually indicated by slight depressions on the 
surface.” 
Then referring to the error committed by Ehrenberg and 
Kutzing, of considering the central and terminal expansions of the 
median band as apertures, Dr. Carpenter adds:—* There can no 
longer be any doubt as to the real nature of these nodules. As 
Professor Smith has justly remarked, ‘the internal contents of 
the frustule never escape at these points when the frustule is sub- 
