I N T R O D U C T I O N .
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S e c t io n I.
H a b it a t s a n d G e n e r a l A p p e a r a n c e oe t h e D ia ïo m a c e æ .
T h e Diatomaceæ are readily distinguislied from the Desmidieæ,
the Palmellaceæ, and other nnicellnlar Algæ, by the possession of
an epidermal covering of silex, which renders their forms indestructible
by the ordinary agents of decomposition. They are
all exceedingly minute, and require the human eye to be aided
by the various appliances of the optician’s art, that their beautiful
forms may become objects of admiration and study. They
inhabit the sea or fresh water, but the species peculiar to the
one are never found in a living state in the other locality,
though there are some which prefer a medium of a mixed
nature, and are only to be met with in water more or less
brackish. The latter are often found in great abundance and
variety in districts occasionally subject to marine influences, such
as marshes in the neighbourhood of the sea, or the deltas of rivers,
where, on the occurrence of high tides, the freshness of the water
is affected by percolation from the adjoining stream, or more
directly by the occasional overflow of its banks. Other favourite
habitats of the Diatomaceæ are stones of mountain streams or
waterfalls, and the shallow pools left by the retiring tide at the
mouths of our larger rivers. They are not however confined to
the localities I have mentioned,— they are in fact almost ubiquitous,
and there is hardly a roadside ditch, water-trough or