OF SOME NORTH WORCESTERSHIRE POOLS. 429 
V. SPECIES OF PARTICULAR INTEREST. 
A noteworthy feature of the heleoplankton collections was the number of 
alga that were either very rare or not commonly found. It will be seen 
from the map that the area in which the collections were made is of quite 
limited extent, and includes but a small part of the basin of the River Stour. 
Nevertheless, in spite of the limited area and the single collection from 
each pool, many alge previously recorded for solitary spots in the British 
Isles, or for the Continent, were found to occur sometimes in considerable 
quantities. Up to the present the majority of collections of phytoplankton 
have been confined to the sea or to large sheets of fresh water, while many 
of the smaller pools scattered over various parts of the country have been to 
a great extent neglected. It seems probable that an investigation of the 
heleoplankton of small pools over a large area would reveal the widespread 
existence of many alge which are now considered rare. 
The following species of special interest were found :— 
1. Microcystis ocHRACEA (Brand), Forti, in De Toni, Syll. Alg. v. 86. 
(Polycystis ochracea, Brand, Zur Algenfl. der Wurmsees, 1898, p. 200.) 
(РІ. 34. figs. 1 & 2.) 
Diameter of cells 10 д; length of colony may be 120 y. 
This alga occurred in large quantities in Spring Grove Lower Pool. The 
mucus in which the spherical cells are embedded is highly refringent, and 
gives each colony a very strongly marked outline. The colonies often attain 
large dimensions, and are of very irregular form. Each cell contains 
numerous small dark granules, which are now known as * pseudovacuoles ^ 
and were at one time regarded as gas-vacuoles. The organism has been 
recorded from the Wurmsee, Bavaria, from Lake Varano, Italy, and from 
Victoria Nyanza, Central Africa. It has not been previously recorded for 
this country. 
2. Preromonas ACULEATA, Lemm, in Ber. Deutsch. Botan. Ges. xviii. 
(1900) p. 94, t. 3. Е. 11. (PL 34. figs. 3, 4, & 5.) 
Length of cell 2077 u, breadth 20°3 р. 
This interesting member of the Chlamydomonadez occurred in small 
numbers at Spring Grove, mostly in the Lower Pool, but a few were found 
in the Upper Pool. The wing-like prolongation of the cell-wall varies 
slightly in shape. The cell-wall is ornamented with six rounded projections 
arranged in two rows transverse to the axis of the cell. When viewed from 
the side there appear to be three projections, and when viewed along the 
axis there are two. Four pyrenoids are present. The organism has not 
been previously recorded for this country. It has been found also in the 
Stakenbridge Pool, another pool in the Stour basin. 
Other members of the genus found in England are Р. angulosa (Carter), 
