Mr. Thompson on an Alga which colours Ballydrain Lake, 83 



same occasion, consider it as belonging to a genus that had 

 not been recognised as British. Having preserved a quantity 

 of the plant I communicated specimens to Dublin, where on 

 the following month they were seen in Miss Ball's collection 

 by Professor Morren, who was highly gratified to recognise 

 them as the alga discovered by himself in Flanders, and for 

 which he constituted a new genus — Aphanizomenon — that was 

 announced to the Royal Academy of Brussels in the prece- 

 ding month of December. 



M. Morren observes in reference to this species — "Vers le 

 milieu du mois de Mai jusqu'au mois de Juillet, on trouve des 

 etangs, des mares, des bassins, qui environnent les maisons 

 de campagne en Flandre, dont l'eau offre des flocons d'un 

 vert blanchatre et de la grosseur qui varie de celle d'un 

 petit pois a celle d'un melon. Ces flocons, qui paraissent nua- 

 geux de loin, sont places a distance les uns des autres ; on les 

 dirait immobiles, mais vus de plus pres, ils jouissent d'une 

 veritable locomotilite, qui permet de les rencontrer a toutes 

 les hauteurs dans l'eau. J'en ai observe cette annee encore, 

 prodigieuse quantite a Gentbrugghe, pres de Gand." 



In Ballydrain Lake I have, both in 1838 and 1839, remarked 

 its presence in very calm days, for it is only at such times vi- 

 sible, during the months of July, August, and September, 

 and then it appears in the most sheltered creeks only, floating 

 in patches of various dimensions. 



Under the separate heads of ft Organologie" and " Physio- 

 logic de 1'Aphanizomene," highly interesting details, which I 

 must content myself with referring to, will be found in M. 

 Morren's Memoir ; as however this may not be accessible to 

 all British botanists, it seems to me desirable that the follow- 

 ing at least should be copied from it*. 



" APHANIZOMENON f. 

 Filamenta shnplicia, cylindrical flexilia, membranacea, vitrea, 

 articulaia, articulis in lamellis plants, apice laciniatis, 

 coadnatis, rectis aut hie et illic inflatis, materia viridi 

 farctis, oscillantihus, sponte dissilientibus. 



* I have only seen a separate copy of this memoir which was sent by the 

 author to Miss Ball. It was printed at Brussels in 1838, hut whether as 

 part of the Transactions of the Royal Academy of that capital is not stated, 



f De »(pu,'M^6(/.vj6u i qui se dissipe. 



