.248 BRITISH SEA-WEEDS. 



sion ; but this course has not been sanctioned by the 

 best authorities, who have considered that structure and 

 mode of fructification are characters of greater value 

 than colour. 



Genus CVII. PORPHYRA. 



Frond a flat, irregularly-shaped, very delicate purple 

 membrane. Fructification, purple spores, arranged in groups 

 of four throughout the frond. — Porphyra, from the Greek 

 porpJiureos, purple. 



There is a wide divergence of opinion as to the num- 

 ber of species into which this genus should be divided. 

 Kiitzing has described sixteen ; but other writers treat 

 many of these as only varieties. Hitherto two at least 

 of the forms found on our coast have been considered 

 to be distinct; but Dr. Harvey's most recently ex- 

 pressed opinion is that they are not, and that "if we 

 contend for two species, with equal justice we might 

 make half-a-dozen." I think so too, and I therefore 

 unite both forms under one name. 



Porphyra vulgaris. The common Porphyra. 



Fronds of very variable size and shape, some ribbon-like, 

 and attached at the base, others spreading from a central 

 root, and more or less divided into irregular, ragged seg- 

 ments ; their usual colour is a dark purple, which is de- 

 rived from the fruit-bearing cells, that extend over the 

 whole fronds. Specimens not in fruit are of a blackish- 

 green hue. 



The different forms of this species are all, I believe, 

 the results of variations of season and situation. In 

 early winter, and near high- water mark, the narrow, 



