240 ALG.E CHLOROSPERMEjE. [Porphyra. 



8. O. corium, Ag. Leathery Oscillaloria. Stratum thick, 

 subcoriaceous, opaque, dull-brownish, streaked with pale-green ; 

 filaments yellowish, slender ; striae indistinct, distant. Harv. in 

 Hook. Br. FL v. ii. p. 377. 



On the rocky bottoms of alpine rivulet?. 



9. O. subfusca, Vauch. Brownish-green Oscillatoria. Stra- 

 tum dull greyish brown, somewhat streaked with a green shade, 

 soft, void of tenacity ; filaments very slender, hyaline, straight ; 

 striae conspicuous. Harv. in Hook. Br. Fl. v. ii. p. 377. 



On rocks and stones in sub-alpine rivulets. 



10. O. ochracea, Grev. Fragile Oscillatoria. Forming float- 

 ing, cloud-like, very fragile masses of an ochrey colour ; fila- 

 ments scattered, very slender, acicular. Harv. in Hook. Br. 

 Fl. v. ii. p. 378. — Conf. ochracea, Dillw. t. 62. 



In boggy pools. Agardh considers this to be some other species in 

 decay ; in which opinion he is, perhaps correct. 



Tribe XX. ULVACE^E. 



Plants growing in the sea, in fresh water, or on damp ground, 

 of a membranaceous or gelatinous substance and simple structure. 

 Frond either a tubular or flat, filiform or expanded membrane, 

 or a gelatinous amorphous mass ; hyaline, or, owing to the pre- 

 sence of fructification, of a green, purple, or pinkish colour. 

 Fructification: minute green or purple granules, scattered through 

 the frond, or arranged in fours, or in many monilif or m, filamen- 

 tous scries. To this family (as understood by Dr. Greville), I 

 have ventured to add the " Nostochinm" of the British Flora. 

 Any one acquainted with these plants must be aware, that 

 though there is much apparent difference between the extreme 

 genera ( Porphyra and ProtococcusJ, yet the line, even of ge- 

 neric distinction, cannot clearly be defined in the medial ones. 

 Thus, Ulva insensibly passes to Tetraspora, this into Palmella, 

 this again into Flaimatococcus, which is scarcely different 

 from Protococcus. Nostoc, I allow, in its moniliformly disposed 

 sporules, presents a somewhat different organization, and per- 

 haps, notwithstanding its strong affinity to Palmella, it might 

 with propriety form the type of a distinct family. In structure 

 it appears exactly intermediate between Palmella and Oscilla- 

 toria. 



76. Porphyra. Ag. Porphyra or Purple Laver. 



Frond plane, exceedingly thin, and (owing to the fructification) 

 of a purple colour. Fructification : 1. scattered sori of oval 



