illustration of their minute size it may be mentioned that a single drop
of water from the saucer of a flower-pot at Hertford, mounted as a
microscopic slide, was found to contain 200,000 separate frustules of
_Achnanthes subsessilis_, and it was estimated that these occupied only
one twenty-fifth part of the drop. Both species of _Chlamidococcus_ (the
old genus _Protococcus_), _C. pluvialis_ and _C. nivalis_ occur; and
the pretty _Volvox globator_ has frequently been found.

Of _the Lichens_ much less is known, only sixty-seven species having
been recorded. The most noteworthy are _Calicium melanophæum_, found on
fir-trees in Bricket Wood; _Peltigera polydactyla_, on moss-covered
ground in Oxhey Woods, Watford; _Lecanora phlogina_, in the Tunnel
Woods, Watford; and _Pertusaria globulifera_, on trees in the same woods
and also in Bricket Wood. As woods in the vicinity of Hertford and of
Watford only have been searched for lichens, our list ought to be
largely increased by investigation in other parts of the county.

Of _the Fungi_ our chief knowledge is derived from lists of species
collected at Fungus Forays of the Hertfordshire Natural History Society
and from records of the Mycetozoa by Mr. James Saunders. The number of
species recorded for the county is 735, of which fifty-eight are
"myxies". Of the Hymenomycetes, or mushroom-like fungi, some very
noteworthy finds have been made, nearly all at Forays of the county
society. They include two species new to Britain, _viz._, _Agaricus
(Nolania) nigripes_, found in Aldenham Woods, Watford, and _Ag.
(Hypholoma) violacea-ater_, in Gorhambury Park, St. Albans (by the
present writer). Hertfordshire has also furnished the second British
records for _Ag. (Lepiota) gliodermus_ (Broxbourne Woods), _Ag.
(Leptonia) euochrous_ (Ashridge Woods), _Ag. (Psathyrella) aratus_
(Sherrard's Park, Welwyn), and _Paxillus Alexandri_ (Hatfield Park),