Ceramiea.'] ptilota. 205 



Eev. Gilbert Laing from Orkney. The largest specimen 

 we ever saw was in Broclie's collection of Algae^ now the 

 property of Professor Walker Arnott. It measui'es twenty 

 inches in breadth by sixteen in height. 



2. Ptilota sericea, Gmelin. (Plate Til. fig. 28^ natural 

 size ; to the left a magnified plumule.) 



Hab. On the perpendicular sides of rocks, between tide- 

 marks. Earely on the stems of Fiiciis serratus. Perennial. 

 Summer and autumn. Yery common. Pound also on the 

 Atlantic shores of Europe and on the east coast of North 

 America. 



The difference betwixt this and P. plumosa may be seen 

 by comparing the frond and pinnule in Plate YIII. with the 

 frond and pinnule in Plate YII. It long ranked only as a 

 variety of P. phmiosay and we are glad that it is restored to 

 its original dignity of a species, the Fiicus sericeus of Gmelin. 

 We think it well entitled to the rank. P. plumosa is never 

 found on rocks, always on Laminaria digitata; P. sericea is 

 almost always found on rocks, and never that I know of on 

 L. digitata, Ptilota plwiiosa is greatly infested by Mem- 

 branipora pilosa : tliis zoophyte is scarcely ever seen on P. 

 sericea, though in spring it is often almost covered by Stria- 

 tella arciiata, a parasite which I have never observed on P. 



