196 THE ESSEX NATURALIST
Pediastrum biradiatum Meyen Rare.
Golenkinia radiata Chodat (S., 1962)
Rare. Cells from the William Girling Reservoir were studied
in culture.
Micractinium pusillum Fresenius (S., 1962)
Frequent, especially in spring, when it is sometimes an im-
portant constituent of the plankton. In April and May 1958
numbers of colonies ranged from 100 to 700 per ml.
Oocystis sp.
The four-celled colonies resembled those of O. lacustris, but
identification of the species of this genus is difficult.
Lagerheimia wratislaviensis Schroder (S., 1962) Occasional.
Chodatella ciliata (Lagerh.) Lemm.
Seen once, Rye Meads, August 1959.
Chodatella subsalsa Lemm. (B. & S., 1962)
Occasional in the Lee. Isolated into culture from the William
Girling Reservoir, which is fed from the river. First British
record.
Chodatella quadriseta Lemm. (S., 1962)
Occasional in summer. The planes of the pairs of spines at
each end of the cells from the Lee were always at right angles to
one another.
Tetraedron minimum (A. Br.) Hansg. Occasional.
Tetraedron incus (Teil.) G. M. Smith Rare.
Tetraedron caudatum (Corda) Hansg. Occasional.
Polyedriopsis spinulosa Schmidle (B. & S., 1962)
Occurred in September 1959, when it was cultured and zoospore
formation was seen. First British record.
Scenedesmus acuminatus (Lagerh.) Chodat Frequent.
Scenedesmus quadricauda (Turp.) Breb.
Frequent. A small variety of this species was also frequent.
This had additional smaller spines between the two larger ones
on the end cells.
Scenedesmus opoliensis Richt. Rare.
Other unidentified species of Scenedesmus were also present.
Actinastrum hantzschii Lagerh. Occasional.
Crucigenia rectangidaris (Nag.) Gay Occasional.