cistern which will not reward a search, and fnrnish specimens
of the tribe.
The indestructible nature of their epiderm has also served to
perpetuate the presence of these forms in numerous localities,
from which their living representatives have long since disappeared.
Districts recovered from the sea, in the present or
other periods of the earth’s history, frequently contain myriads
of such exuviae forming strata of considerable thickness. Such
deposits have been found in Bohemia, in the neighbourhood of
Berlin, in various districts in Italy, and in several of the American
States. The city of Richmond in Virginia is said to be built
upon a stratum of Diatomaceous remains 18 feet in thickness,
and extensive tracts in the Arctic Regions have been found
covered with similar relics of a former vegetation.
Nor are we without examples, though on a less extensive
scale, in onr own country. The ancient site of a mountain lake
in the neighbourhood of Dolgelly, localities of a somewhat similar
kind near Lough Island-Reavey, in Down, and Lough Mourne,
in Antrim, have furnished large supplies of some of the forms
described in tbe present work. Several deposits of a like kind
liave been met with in Scotland, and have also contributed to enrich
the present volumes. The extreme minuteness of the organisms
which have furnished such remains, and the hardness of their
material, have rendered the substance formed by tbeir aggregation,
a useful agent in the mechanical arts, in which it has been
employed to confer a polish upon hard surfaces. It is from this
circumstance that the material known as Tripoli derives its value
as a polisher of metals ; and the DolgeUy deposit has to some
extent been employed for a similar purpose.
One of the most singular instances of the preservation of
Diatomaceous forms occurs in regard to Guano, so largely imported
as a manure from Peru and Africa. This material is
found to contain a large number of the siliceous coverings of
these minute organisms, which have been swallowed by the marine
birds frequenting the spots from which the Guano is procured,
have survived the process of digestion to which they
were at first submitted, and the ages during which they have
been imbedded in decomposed or decomposing matter, and now
serve by the beauty of their forms to delight the microscopist,
and by the property which their presence imparts to the Guano,
to contribute to the fertility of onr pastures and the growth of
our cereals. It is well known that the latter plants contain a
large amount of silica in the structures of their stem and leaves ;
it is therefore probable that the value of this manure may in
some degree depend upon the presence of these minute organic
remains, which may thus confer upon the Guano a quality rendering
it eminently conducive to the healthy growth of such
crops.
When occurring in strata of a fossil or subfossil character, as
the deposits of DolgeUy, Richmond, and Lough Mourne, the
epiderms of the Diatomaceae appear as a white or cream-coloured
powder ; but their living masses present themselves as
coloured fringes attached to larger plants—or forming a covering
to stones or rocks in cushion-like tufts—or spread over
their surface as delicate velvet— or depositing themselves as a
filmy stratum on the mud— or intermixed with the scum of
living or decayed vegetation floating on the surface of the water.
Their colour is usually a yellowish brown of a greater or less
intensity, varying from a light chestnut in individual specimens,
to a shade almost approaching black in the aggregated masses.
Their presence may often be detected without the aid of a microscope,
by the absence, in many species, of the fibrous tenacity
which distinguishes other plants : when removed from their
natural position, they become distributed through the water,
and are held in suspension by it, only subsiding after some little
time has elapsed.
The frustules of the Diatomaceæ, as the individual organisms
have not inaptly been denominated, are either free, adherent, or
variously aggregated : in MtzscMa, Navícula and others, we have
the frustules absolutely free ; in Epithemia, Cocconeis, &c., they
are usually adherent. One mode of aggregation is that of a
ribbon-like filament of indefinite length, as in Eragillaria, &c. ;
another, that of a zigzag chain, in which the frustules cohere
only by their angles, as in Grammatophora, Eiatonia and others.
In some species the Diatom is provided with a gelatinous
pedicel or stipes by which it is united with other frustules, and