158 CERAMIACEJE. 



which they are formed, or they project beyond the surface, 

 like little pimples. In most cases they are tripartite, in a 

 few cruciate, but I do not remember any transversely-zoned 

 tetraspores among the plants of this order. 



In several exotic genera the frond assumes the appearance 

 of a network, owing to the branches and ramuli continually 

 growing into each other, or anastomosing. Some of these 

 network genera (as Halophlegmd) have no certain form, but 

 resemble crimson sponges, variously lobed and sinuated ; 

 others (as Thuretid) are perfectly symmetrical, their branches 

 and raomli arranging themselves like the costae and nerves of 

 a skeleton leaf, and growing into perfectly defined fronds. 

 One of these network ceramioid genera is found in the Adri- 

 atic, and may yet be discovered by some fortunate dredger 

 in the south of England ; the rest are natives either of the 

 tropics or of Australia. 



Most of the British genera, and several of the species, are 

 widely dispersed. Ceramium rubrum grows almost where- 

 ever marine plants will grow, from high arctic to high ant- 

 arctic latitudes ; and Spyridia jilamentosa is dispersed 

 through all the warmer parts of the sea, and in Britain finds 

 its northern limit. None of this order have been employed 

 in the arts, unless the composition of sea-weed pictures, for 

 which purpose they are great favourites, be considered to 

 entitle them to the honour due to useful plants. 



SYNOPSIS OF THE BRITISH GENERA. 



I. PTILOTA. Frond compressed, inarticulate, distichous, 



pectinato-pinnate. Favellce pedunculate, involucrate. 

 [Plate 22, A.] 



II. MICROCLADIA. Frond filiform, inarticulate, dichoto- 



mous. Favell(B sessile, involucrate. [Plate 22, B.] 



III. CERAMIUM. Frond filiform, articulate, dichotomous ; 

 the joints opake. Favella sessile, mostly involucrate. 

 Tetraspores mostly immersed. [Plate 22, C.] 



IV. SPYRIDIA. Frond filiform, inarticulate ; the branches 

 clothed with minute, setiform, articulated ramelli. 

 Favella pedunculate, involucrate. Tetraspores ses- 

 sile on the ramelli. [Plate 22, D.] 



