76 RHODOMELACE.E. 



to a leafy form that we possess, is replaced in the more ge- 

 nial waters of the Mediterranean by the delicate Dictymenia, 

 in which the surface of the leaf is composed of large cells ; 

 and, pursuing our course to warmer regions, this form gives 

 place to Amansia, whose leaves are still more delicate, 

 with their cells arranged in transverse lines, each cell exactly 

 of the same length as its neighbour cell, and regularly twelve- 

 sided. Such fronds resemble a fine piece of mosaic pave- 

 ment, and exhibit the areolated structure in its greatest per- 

 fection. In several of the leafy genera, the leaves are 

 nerveless, in others nerved, and in some traversed by a sys- 

 tem of branching veinlets, that spread through the whole 

 substance. In one or two they are thick and fleshy, contain- 

 ing large chambers filled with mucus. In many they are 

 proliferous, the new leaves springing from the disks or mar- 

 gin of the old. The net-work fronded, such as Claudea, 

 have been already noticed. The filiform kinds, to which be- 

 long nearly the whole of the British species, are either arti- 

 culated, or furnished with an articulated axis coated with a 

 stratum of small, irregularly formed, polygonal cells. They 

 are all constructed on the same plan : there is a central, arti- 

 culated filament, usually devoid of colouring matter, and 

 round this filament is ranged a circle of elongated cells, of 

 equal length, to which the name siphons or tubes has been 

 given. These whorls of cells, placed one above the other, 

 form the filiform frond ; and the points of the connection 

 of their ends, the joints, when the frond is articulated. When 

 there is no visible articulation it exists in the centre of the 

 frond, but is concealed from view by a coating of cells, 

 of greater or less thickness, as in Rhodomela, or is partially 

 visible, as in Rytiphl&a. 



Many of the Rhodomelacea are of a brown-red, and some 

 of them of a full brown colour, and nearly all become darker 

 in drying. Some that are rose-coloured, as Pol. byssoides, 

 when living, become quite dark after they have been dried, and 

 others turn completely black, a peculiarity which has sug- 

 gested the ordinal name (signifying red-black). Many dis- 

 charge a quantity of dark coloured, offensive matter, when 

 steeped in fresh water, and several will rapidly decompose 

 when brought in contact with that medium. Others may be 

 kept in it for days without injury, or even with advantage, 

 for by this means the tendency to dry black is lessened. 

 All, except some of the Bostrychice, are strictly marine, and 

 generally grow near low-water mark. Our own Bosirychia 



