240 ALG.E CHLOROSPERMEjE. [Porphyra. 



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

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

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

 Hook. Br. PL v. ii. p. 377. 



On the rocky bottoms of alpine rivulets. 



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. 37S.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. ULVACE7E. 



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 moniliform, filamen- 

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

 have ventured to add the " Nostochince" 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 Protococcus ) ', 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 Flcematococcus, 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 



