I. EOBINSOH—THE DIAT0MACE2E. 
7 
In order to convey some idea of the abundance and variety 
referred to, I may instance a collection I made from the old River 
Rib, near Hertford, in September, 1883. This collection consisted 
of a small quantity of dirty and repulsive-looking scum which was 
floating on the surface of the water, and which I took home in a 
small bottle in my pocket. A subsequent careful examination 
under the microscope enabled me to identify no less than 56 distinct 
species contained in it. Another collection from an isolated pond 
in a field near Rrickendon yielded 26 species, whilst others from 
the river Ash and the adjoining pond at Easneye, made on the 
occasion of the Society’s field meeting there last June, yielded 
44 species. These statements are sufficient to indicate the very 
wide and general distribution of these forms. They may also be 
found in some places in which their presence would appear far less 
probable. Thus in the early part of last October I found in the 
saucer of a flower-pot, containing a large plant of Agapanthus um- 
bellatus which had been standing in my garden during the summer, 
a large quantity of a rare form known as Achnanthes subsessilis, 
to which I have already referred. In the saucer of another 
flower-pot I found a considerable quantity of a totally different 
form known as Nitzschia Amphioxys. I have also found many 
beautiful marine forms in the stomachs of tinned oysters imported 
from America. These have been swallowed by the oysters with 
their food, but being incapable of digestion have remained uninjured. 
Many species are also to be ‘found in guano. These have been 
swallowed by sea-birds with their food, and in consequence of their 
indestructibility have passed through their systems uninjured. It 
is however remarkable that such delicate forms should have escaped 
the action of their powerful gizzards, but probably their minute 
size has saved them. 
Diatoms are also found in a fossil state, and in this condition 
they form strata of considerable thickness in various parts of the 
world. These strata have doubtless been formed in the beds of 
lakes, and consist of the accumulation of the frustules of countless 
generations. Yarious examples of this character exist in Great 
Rritain, as for instance at Dolgelly in Wales, in the island of Mull 
in Scotland, and at Toome Bridge and elsewhere in Ireland. A 
more remarkable instance, however, is that of Richmond in Virginia, 
where the whole city is built upon a stratum of diatoms 18 feet in 
thickness, and one which contains many beautiful forms. Another 
very extensive deposit is found in Bohemia, where it occurs in 
beds of about 14 feet in thickness which supply the fine sand 
used for delicate castings. Another deposit found in Hanover is 
known as Kieselguhr or flint-mud. It has been found to possess 
the peculiar character of absorbing three times its weight of nitro¬ 
glycerine, and is therefore made use of in the manufacture of 
dynamite. The preparation known as “ Stevens’s silicon,” found 
in the State of Nevada and used for polishing jewellery, etc., 
consists almost entirely of the frustules of diatoms. Tripoli and 
rotten-stone, used for polishing, are similarly constituted. It 
