XXII. 
OBSERVATIONS ON DIATOMACEAE FROM THE NEIGHBOUR¬ 
HOOD OF HERTFORD. 
By Isaac Robinson. 
Mead at Hertford, 24 th February, 1887- 
About three years ago I read a short paper on the Diatom'aceae * 
with particular reference to the species found in our own county. 
Since that time Mr. Francis Ransom has contributed a paper on 
the same subject,! which, however, dealt principally with the 
nature and habits of these organisms. It might therefore he con¬ 
sidered that a third paper was superfluous at the present time. My 
apology, however, must he that I have been enabled to make con¬ 
siderable additions to the list of local species, and that some facts 
of an interesting character, in reference to the distribution and 
reproduction of the Diatomaceae, have come under my notice. 
Diatoms are characterised by the possession of silicious epiderms 
which usually consist of two symmetrical valves and a connecting 
hoop, the three parts together enclosing the living substance of the 
organism. This living substance consists of a mucilaginous fluid, 
invested in a membranous sac or covering, in which float the 
granules of endochrome, giving to the living frustule a golden- 
brown colour. This unicellular constitution of the organism ap¬ 
pears to he one of great simplicity, and it never ceases to my 
own mind to be a subject of wonder how it is enabled to produce 
those beautiful frustules which are the delight of the micro- 
scopist; which, whether circular, oval, hexagonal, oblong, or other¬ 
wise, are so marvellously perfect ; and which, with all their 
delicate design and tracery, are formed of a material of so inde¬ 
structible a character as well-nigh pure silica. 
The Diatomacese are found in fresh water, in brackish water, 
and in the sea, much the larger number being fresh-water and 
marine forms. Our own county, as a matter of course, only affords 
examples of the fresh-water species. The number of these found 
in Great Britain, as recorded by Professor Smith, J was 238, and 
some others have, since the publication of his work, been added by 
other observers. The number of species recorded as found in our 
own county up to the present time is about 150, nearly the whole 
of which have been found near Hertford, although many of them 
have been also recorded from elsewhere. A list of the species is 
appended. Two species, which are not included in the list, may 
be mentioned here, for, though they must be regarded as absentees 
from our own county so far as present observation has gone, they 
have been found not far distant from its boundaries. One is Aste- 
rionella formosa , found in the canal of the East London Water 
Works near Lea Bridge, and the other is Gomphonema geminatum , 
found in Epping Eorest. 
* ‘Trans. Herts Nat. Hist. Soc.,’ Vol. Ill, p. 1. 
f ‘ Synopsis of the British Diatomacese,’ 1853-56. 
f lb. p. 197. 
