Marine Algce of Berivick-on-Tweed. 313 



roundish or irregular in outline, on the stems of the host plant, 

 to which they are strongly attached by their entire under 

 surface ; the substance is parenchymatous throughout ; the 

 tetraspores cruciate, or irregularly divided, and mixed with short, 

 stiff, free paraphyses which curl over them in a most character- 

 istic way. The thallus varies greatly in thickness, the vertical 

 filaments consisting of from 12 to 30 cells. Even different 

 portions of the same frond are of very different thickness, the 

 thallus of the Rhododcrmis being much thicker where it grows 

 over a depression in the stem of the host plant. The cells of the 

 thallus are a little longer than broad, being about -009 mm. 

 long, and -008 mm. broad at the base of the frond, '007 mm. 

 broad near the surface. The sori, which are never very 

 numerous, form indefinite dull patches, rather lighter coloured 

 and pinker than the rest of the frond, and scattered very 

 irregularly over its surface. The outline of the thallus is hardly 

 ever to be seen as the Rhododermis usually entirely encircles the 

 stems of the Laminaria for a distance of several inches, and is 

 more or less infested by other Algre, such as Ptilota plumosa, 

 Polysiphonia urceolata, Delesseria alata, Delesseria sinuosa, Euthora 

 cristata, etc. 



In Rhododermis elegans the thallus cells are broader than long, 

 and of uniform thickness in all parts of the frond, in Rhododermis 

 parasitica they are longer than broad, and of different thickness at 

 the base and surface of the frond, the thallus of the latter plant 

 is, moreover, thicker, denser in substance, and darker coloured 

 than that of the former. 



PETKOCELIS, J. Ag. 



Petrocelis cruenta, J. Ag. 

 Spec. Alg. ii., p. 490. 

 Descr. Petrocelis crnenta, Farlow, Mar. Alg. New Eng., p. 115. 

 Fig. ,, „ Le Jol., Liste Alg. Mar., pi. in., figs. 3, 4; 



Farlow, I.e. pi. 14, fig. ] . 

 E.vnicc. ,, „ Crouan, Alg. Finisfc., no. 233. 



Syn. Cruoria pellita, Harv., Phyc. Brit. pi. 117; Ktz., Spec. Alg., p. 



533 (non Fries.) 

 Hah. On rocks from near high-water mark to about half-tide level. 

 Frnit Jan. and Feb. Not. uncommon. Spittal, Scremerston. 

 1 O 



