er THE CHARACTERISTICS OF THE DIATOMACEiE. 37 
CHAPTER V. 
OF THE CHARACTERISTICS OF THE DIATOMACEJE. 
Before concluding my remarks on the Diatomaceze I wish 
to call the attention of the reader to the principal charac- 
teristics by which they are distinguished from other mem- 
bers of the vegetable kingdom. These are their form and 
the curious markings of their silicious coats ; and with 
these, varying as they do in the most extraordinary 
manner, the botanist should make himself thoroughly 
acquainted. 
With regard to the form of the plant, he must not be 
content with a single view of the individual, however 
favourable it may seem. Whichever side is turned towards 
him, when the specimen is first placed under the lens, his 
first object should be to roll and turn it on the slide, in 
such a manner as to expose the other side also to view. 
This is a difficult business with many of the species (those, 
for instance, of Fragilaria) which are propagated in long 
ribbon-like filaments, and which, from the flatness of their 
outline, lie close against the glass. He must be prepared, 
indeed,, to give up a good deal of time, and to exercise no 
little patience, before he accomplishes his object; but 
practice and experience are sure to bring success in the 
end. 
As the language in which authors describe the two 
aspects under which every Diatomaceous Alga ought to be 
viewed is often obscure, the reader may get a good notion 
for himself by the inspection of an ideal figure like that 
which is here annexed, fig. 45. It represents a column or 
cylinder, divided into several parts, and, by naming the 
