AMFBIDINIUM OPERCULATUM AT PORT ERIN. 71 



On the Occurrence o£ AmpJiidinium opercidatum, Clap. & Lach., in vast 

 Quantity, at Port Erin (Isle o£ Man). By Prof. W. A. Herdman, 

 F.R.S., F.L.S. 



(Pl,ATE 8.) 



[Read 1st June, 1911.] 



In ffoino' to and fro between the villag-e of Port Erin and the Biolog-ical 

 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 gets into the habit, in these traverses, of looking closely at the beach 

 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 

 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. 8. 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 awa}-. 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 Amphisbcena *, 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 identify this 

 form as Amphidinium opercidatum, described by Claparede and Lachmann, 

 in 1858, from specimens obtained at Christiansand, Bergen, and a few other 

 places in Norway. 



The published records of AmpJiidinium, 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 : A. crassum, 

 A. rotundatum, and A. longum, which as yet have been recorded from Kiel 



* See postscript at end of this paper. 



