DELESSERIA. 113 



transversely zoned. In all cases they form well-defined soi-i 

 or spots, which either occupy a portion of the surface of the 

 membranous frond, or are confined to spore-leaflets {sporo- 

 phylla), which spring from some part of the frond. This 

 order is chiefly known from the Rhodymeniacea by the defi- 

 nite arrangement of the tetraspores ; but the plants of the 

 latter family are of a coarser texture, and in many instances 

 have a partially fibro-cellular structure. 



The Delesseriacece are not numerous, but they are widely 

 scattered through the temperate and colder latitudes of both 

 hemispheres. Few are tropical, and those of small size. On 

 the shores of northern Europe and of the antarctic lands the 

 finest species occur, some having fronds several feet in length. 

 Where soil and exposure are favourable, some of the British 

 Nitophylla, especially N. pimclatum, attain very large di- 

 mensions. Of Plocamium our common species is found in 

 all cool waters, being equally common at the North Cape of 

 Europe and at Cape Horn, and extending in both hemi- 

 spheres to some 34° of the line ; but it is the only one of the 

 genus to which it belongs whose habits are equally pelagic. 

 All the other species, and there are several, are natives of the 

 southern hemisphere, chiefly of the shores of South Africa 

 and New Holland. 



SYNOPSIS OF THE BRITISH GENERA. 



I. Delesseria. Frond leafy, of definite form, with a 



percurrent midrib. [Plate 15, A.] 



II. NiTOPHYLLUM. Fvond leafy, of indefinite form, with- 



out a midrib (sometimes traversed by vague, vanishing 

 nerves). [Plate 15, B.] 



III. Plocamium. Frond linear, or filiform, compressed, 

 much branched, distichous ; ramuli pectinate, secund. 

 [Plate 15, C] 



I. Delesseria. Lamour. [Plate 15, A.] 



Frond rose-red, flat, membranaceous, with a percurrent 

 midrib. Fructijicotion of two kinds, on distinct individuals : 

 1, hemispherical fubercleii fcoccidia), mostly on the midrib, 



I 



