THE ESSEX FIELD CLUB. 239
lecting bottle without the intervention of a net or other filtering appliance,
and without any preservative being added, as it was most important
that the organisms obtained should be examined alive. For a similar
reason the sample should be centrifuged as soon as possible after col-
lection. Very small quantities of water sufficed as a rule to give a fair
idea of the nannoplankton organism; present and their relative abundance.
The glass tubes commonly supplied with a small centrifuge holding about
15 cc., were ample for most fresh-water investigations, and Mr. Scourfield
exhibited a modification of a "haematocrit" head for the centrifuge
which he had had made to carry elongated vase-shaped tubes, holding
only 11/2 cc. These, he said, usually gave good results for small pieces of
water such as ponds, but would scarcely be sufficient for the examination
of large lakes in which the organisms were as a rule not so relatively
abundant. After the actual centrifuging for a minute or two at speeds
ranging up to as much as 10,000 revolutions per minute with the "hae-
matocrit" head (two separate centrifugings being recommended for each
sample, one at a comparatively low speed and one at the highest speed
obtainable), all the water except a minute drop at the bottom should
be pipetted off with a fine "Rousselet" pipette. This drop should be
sucked up and expelled a few times with the pipette in order to detach
any organisms adhering to the bottom of the tube and then a portion
or the whole of it transferred to a glass slip, live-box or compressor and
placed under the microscope for examination by the usual methods.
Ths tiny organisms so obtained were found to belong mainly to the
groups of the Bacteria, Schizophyceae, Desmids, Diatoms and Chlorophyceae
among the plants and Heliozoa and Flagellata among the animals. They
ranged in size from about 1-1000" downwards to the smallest Bacteria,
and, strangely enough, very few, if any, seemed to surfer from the centri-
fuging process. Their very minuteness and close approximation to
the specific gravity of water was no doubt a protection to them against
the centrifugal pressure. Many of the forms appeared to be undescribed,
but it was difficult to say at present how far this was really the case. Some
were certainly new to science, and one which had been obtained from a
pond on Leyton Flats had recently been recorded by Prof. G. S. West, as a
new species and type of a new genus.
The proof of the great abundance of these very minute forms in most
waters had come almost as a revelation, and had led to an increased
appreciation of the important role played by living organisms in ponds
and lakes and also in the sea. The significance of the nannoplankton
as a source of food for many Entomostraca, Rotifera, &c., and thus in-
directly for the still higher and larger aquatic creatures, had been amply
demonstrated. And thus, as was indeed usually the case, a new method
of investigation was found to lead to an altogether wider outlook upon
matters about which all essential facts were supposed to be well known.
Lime-stone Deposit of the River Can.—Mr. Percy Thompson ex-
hibited specimens of an impure limestone deposit formed by the small
river Can, to the northwest of Chelmsford.
The deposit occurs in the bed of the river in various places from above
Pengymill down to the Chelmsford-Roxwell road near the point where
it is joined by the Roxwell Brook, a distance of about l1/2 miles, and forms