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PROGRESS OF MICROSCOPICAL SCIENCE. 
The Mode of Existence of the Dendroid Graptolites. — In a paper on 
the Graptolites of the Arenig Rocks of St. David’s, read before the 
Geological Section of the British Association at Brighton, Mr. John 
Hopkinson mentioned the occurrence of a new species of Callograptus, 
with an entire “ hydrocaulus,” or main stem, and a “ hydrorhiza ” — 
the organ of attachment. This appears as a series of interlacing 
fibres at the proximal end of the main stem, which suddenly widens 
out and becomes more loose or fibroiis in structure as it merges into 
the adhesive fibres. The genus Callograptus is thus proved to have 
been capable of attachment to foreign bodies, and it may therefore be 
inferred that its near allies, Ptilograptus, Dendrograptus, and Dic- 
tyonema, were also provided with hydrorhiza}. Ptilograptus and 
Dendrograptus being thus, in general form, in mode of growth, and 
most probably also in structure, essentially similar to our recent 
Hydroida ; Callograptus and Dictyonema differing only in their 
branches, being more or less regularly connected together by 
transverse processes. For this species the name C. radiculatus is 
proposed, to distinguish it as the first in which these radicular fibres 
have been seen. 
The Nature of Diatom Structure. — Some researches on this point 
have been lately carried on by Professor Weiss, of Lemberg, who 
regards the silicious envelope as capable of polarizing light, and as 
consisting of a cellulose coat more or less infiltrated with silex. He 
does not consider the individuals one-celled, but finds the valves com- 
posed of cells from • 008 to • 00025 mm. in diameter. These cells are 
furnished at their centres with papillae, which appear as stria} under 
low powers, and as moniliform markings under high powers. The 
large cavity between the frustules is regarded as equivalent to the 
embryo-sac of higher plants, and the formation of new individuals has 
been observed within it. An alternation of generation is indicated by 
the observations made. 
The True Markings of the Test Podura Seale. — An interesting 
paper on this subject has appeared in the July number of the Ameri- 
can ‘ Lens.’ From the latter journal we take the following account, 
which, though somewhat long, is incapable of being abstracted. In 
the first place, says the author, Dr. Arnold, of New York, I will 
briefly state my views, as expressed in June, 1871. There are, I 
believe, some species of Podura that are scaleless, and are clothed with 
hairs — compound hairs ; that is, the hairs are covered by projections, 
usually in considerable number at one end, where they form a brush- 
like arrangement. In fact, all the sj>ccies of Podura that I have met 
with possess analogous hairs to a greater or less degree. It is, then, 
my purpose to try and establish some points of similarity in structure 
between the hairs and scales themselves. If we agree that these com- 
pound hairs are covered by epithelial scales, and are hairs of other 
