2 
I. ROBINSON-THE DIATOMACEiE. 
definitely observed until so recently as 1745. In that year a Mr. 
William Arderon, E.R.S., discovered in a ditch in Norwich what 
he termed the “ oat-animalcule,” probably one of the Navicula 
now so well known. A few other forms were subsequently dis¬ 
covered, until, in the year 1824, Agardh published his ‘ Systema 
Algarum,’ and recorded 49 species comprised under 8 genera. 
With the subsequent rapid advance in optical science, and the 
great improvements effected in the construction of the microscope, 
the additions made to the list of known species were very nume¬ 
rous, and at the present time those of Europe alone are to be 
reckoned not by hundreds but by thousands. The subject has also 
proved itself to be of so interesting a character to microscopists that 
a large amount of literature has grown up in reference to it. The 
best work descriptive of British species is the synopsis of Prof. 
Smith, which however was published from 25 to 30 years ago, and 
must therefore now be considered incomplete in consequence of the 
great number of species which have since been discovered. 
It was for many years a disputed point whether the Diatomacese 
should be classed in the animal or in the vegetable kingdom. The 
free condition of many of the forms, and their distinct power of 
independent motion, seemed to the earlier observers to indicate their 
animal character. The present generally-received opinion, how¬ 
ever, formed after a much more extended and careful study, is that 
their proper place is in the vegetable kingdom. They are therefore 
now considered to be minute plants, and they are classed amongst 
the Cryptogamia, forming an order of the sub-class Algse. 
The distinctive characteristic of these plants is the possession of 
an almost indestructible siliceous external covering or shell which 
is usually termed a frustule or valve. These frustules exist in 
pairs, thus forming a protection to the living cell of the diatom, 
which -is placed between them. They may for the sake of illustra¬ 
tion be compared to the shell of an oyster, or other bivalve, the 
body of the oyster representing the living cell of the diatom, and 
the shell the two frustules. The diatom, however, does not possess 
a hinge-like connecting membrane as the oyster does, but the two 
frustules are connected round their edges by a band of silica corre¬ 
sponding to the width of their separation. Apertures are provided 
in this flinty covering for the necessary communication with the 
surrounding water. 
The great interest of the Diatomaceae to the microscopist consists 
in their infinite variety of form and in the beautiful and wonder- 
fully-delicate markings which are found upon them. Their inde¬ 
structibility also confers upon them a special value, as they may in 
consequence of it be preserved for reference without deterioration, 
and they supply some of the most beautiful objects we possess for 
microscopic examination. Their shape is exceedingly various. 
Some are perfectly circular in form, others oval, triangular, square, 
many-sided, crescent-shaped, or boat-like. Some are perfectly 
straight and needle-like, and far sharper than any needle fashioned 
by human hands. Others are curved or sigmoid, the curves taking 
