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algfe ; natural order, diatomaceae ; plant, a frustule, consisting of 
a unilocular, or imperfectly septate cell, invested witli a bivalve 
silicious epidermis ; gemmiparous increase by self-division, during 
•which process the cell secretes a more or less silicious connecting 
membrane; reproduction by conjugation, and formation of 
sporangia. 
Before proceeding further, I may mention that the desmidaceous 
and diatomaceous cells are termed frustules, and -when that term 
is used, a cell is to be understood. The diatomacese are distin- 
guished from all other species of algas by the power of eliminating 
from the surrounding water the silex held in solution. This power 
exists in a high degree in the diatomacese ; and when we consider 
that ordinary water contains but an infinitesimal proportion of 
silica, it must excite our astonishment that a minute protoplasmic 
mass, enclosed in a cell, should possess the power, not only to 
eliminate so small a quantity, but also unmixed with other matter 
held in solution with it, and this not deposited in an amorphous 
state, but forming elegant designs in or on the surface of the 
valve. Surely chemical action is not sufficient to account for it, and 
we are compelled to acknowledge that even as in the highest 
organisms, so also in the lowest, there exists a mysterious principle 
called life, defying our utmost efforts to detect or control it ; it is as 
much beyond the j-ower of man to make a diatom, as to make the 
gigantic oak. 
Owing to the indestructible nature of the silicious frustule, 
diatomacese are often found in a fossil state, forming strata of con- 
siderable thickness. The city of Eichmond, Virginia, is built 
over a stratum of diatomaceous remains eighteen feet in thickness ; 
this deposit, according to Professor Eogers, belongs to the Miocene 
period, and so far as I am aware, it is at this period diatoms made 
their first appearance. Ehrenberg, in his Mikrogeologie, figures a 
form as occurring in the chalk which he calls a diatom, belonging 
to the genus Navicula, but it certainly does not belong to that 
genus, nor does it possess any diatomaceous characteristics. 
The sudden appearance of these organisms in tlie Miocene 
period has always struck me as being remarkable, and the majority 
of tlie genera and species found in these deposits liave never 
ceased to exist up to the present time ; in fact, scarcely a fossil 
genus but has its representative still living. Some genera of 
