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CULTIVATION OF DIATOMS. 43
Lacustrine Deposits. These deposits are produced in lakes, ponds,
and rivers. Strictly speaking, they
deposits, since the greater part
are not fossils but rather recent
of the diatom forms contained in them
are identical with living species. They, however, sometimes belong to
the tertiary and also quaternary periods.
These deposits are pulverulent, and when dry they are remarkably
light. They are either quite white or grey, according to the quantity
of organic matter which they contain, and have been said to resemble
powdered starch. Their dampness when fresh usually deepens their
colour which becomes lighter as they dry. These should be gathered
in exactly the same way as marine deposits. As these beds are seldom
of any great extent (they often become soon obliterated or covered
up) it will be well to secure a good supply of the material whenever
the opportunity presents itself. If any shell, wood, or other organic
remains be found dispersed through the deposit, or overlying or
beneath it, they should also be collected, and their position recorded
on the label.
Deposits of fossil diatoms are numerous, and are found in very
various localities. The most ancient is that found in 1878 by Mr.
Shrubsole, of Sheerness-on-Sea, in the London clay, which belongs to
the Lower Eocene (Tertiary period). Some of the diatoms found by
Mr. Shrubsole are encrusted with pyrites, and some have even been
transformed into pyrites. The number of species found is rather large,
the most common being Coscinodiscus perforatus (?)
minor. (?) (')
5. Cultivation of Diatoms.
and Coscinodiscus
Every diatomist has at some time or another made diatom cultiva-
tions, that is to say, has tried to keep alive the species which he has
collected on his excursions, or which may have been accidentally
developed in the jars or aquariums of his laboratory. But the
attempt to keep any species alive for a protracted period rarely succeeds,
and then only under certain circumstances, which cannot be fully
ascertained. The longest cultivation, we believe, which has hitherto
succeeded is one which we have in our own laboratory, dating back
to 1886, since which time some Navicula didyma and Amphora duplex
have continued to multiply.
(
J
)The Diatoms of the London clay, by W. H. Shrubsole, F.G.S., with a list of
species and remarks by F. Kitton, Hon. F.R.M.S., J.R.M.S. (1881), p. 381.