88 THE LITTORAL ZONE. 



occurrence of favourable aspects on shores composed 

 of sandstone, or of clay-slate and the colour of these 

 rocks, render such shores the most prolific in species. 



I shall now take a rapid survey of the vegetation 

 which characterises what is termed the littoral zone* or 

 that belt of rock or shingle which extends from high- 

 water to low-water-mark. Within this space a large 

 proportion of the sea- weeds of our latitude is produced; 

 and the remainder, with the exception of a few sti'ag- 

 glers that extend into deeper water, occur within the 

 limit of two, or, at most, of four fathoms beyond the 

 lowest water of spring-tides. 



Sea-weeds are usually classed by botanists in three 

 great groups, each of which contains several families, 

 which are again divided into genera; and these, in their 

 turn, are composed of one or many species. The num- 

 ber of species as yet detected on the British coasts is 

 about 370, and they are grouped into 105 genera. I 

 cannot, in this place, enter into the niceties of classi- 

 fication to which botanists resort in working out the 

 history of these plants, but must confine myself to the 

 general features of the great groups, and their distribu- 

 tion. Taken in the order in which they present them- 

 selves to us on the shore, and limiting each by its most 

 obvious character, that of colour, we may observe : that 

 the group of Green Sea-weeds (Chlorospermece) abound 

 near high-water-mark, and in shallow tide-pools within 

 the tidal limit ; that the Olive-coloured (Melano- 

 spermeoe) cover all exposed rocks, feebly commencing at 



* See Prof. Edward Forbes, in Geol. Surv. Memoirs. 



