AMPHIDINIUM OPERCULATUM AT PORT ERIN. 289 
On the Occurrence of Amphidinium operculatum, Clap. & Lach., in vast 
Quantity, at Port Erin (Isle of Man). By Prof. W. A. H ERDÁTAN, 
F.R.S., F.L.5. 
(PLATE 18.) 
( Read 1st June, 1911.] 
lw going to and fro between the village of Port Erin and the Biological 
Station, during the recent Easter vacation, I had occasion to take a short cut 
across the sandy beach at least twice and sometimes six times in the day. 
One gt 
when the tide is out, on the chance of seeing something of interest cast up. 
On April 7th, I noticed a new and quite unusual appearance on the sand at 
or a little above half-tide mark. The hollows of the ripple-marks and other 
‘ts into the habit, in these traverses, of looking closely at the beach 
slight depressions formed by the water draining off the beach were occupied 
or outlined by a greenish-brown deposit which in places extended on to the 
level so as to discolour patches of the sand (see PI. 18. fig. 1). 
Here the deposit remained, more or less, for a month—waxing and waning, 
sometimes increasing in a tide, say, roughly tenfold, and at other times 
apparently disappearing for a day or two and then re-appearing either on the 
same part of the beach, or it might be a few hundred yards away. At one 
time it discoloured a continuous stretch of sand about 50 yards long by 
5 yards in breadth just below high-water mark, and was noticeable from some 
distance away. 
At the first glance I supposed the appearance was caused by a deposit of 
Diatoms, but on taking a sample to the laboratory, microscopic examination 
showed that although a few diatoms (including Navicula Amphishena*, or a 
closely allied form) were present, the deposit was formed almost wholly of 
enormous numbers of a very active little Peridinian or Dinoflagellate of a 
bright yellow colour. More careful investigation enabled me to identifv this 
form as Amphidinium operculatum, described by Claparéde and Lachmann, 
in 1858, from specimens obtained at Christiansand, Bergen, and a few other 
places in Norway. 
The published records of Amphidinium, however, do not give the impression 
that it is a common or abundant organism. The latest comprehensive work 
on such forms—the article on Peridiniales, by Paulsen, in the * Nordisches 
Plankton’ (Kiel, 1908)—recognises 4 species of Amphidinium : А. crassum, 
А. rotundatum, and А. longum, which as yet have been recorded from Kiel 
* See postscript at end of this paper. 
[11] 
LINN. JOURN.—BOTANY, VOL. XL. ЗЕ 
