XXVI INTRODUCTION. 



or Regions of Depth. By this plirase is understood the 

 several belts or spaces margining the land, or occupying 

 the floor of the sea, distinguished from each other by the 

 presence of peculiar features dependent on arrangements of 

 their animal and vegetable inhabitants. 



The highest of these belts is the space between tide- 

 marks, an interval of very great importance in the marine 

 Fauna of our Islands, and inhabited by numerous peculiar 

 species" of plants as well as animals. It is termed the 

 Littoral Zone. Its features vary with the geological or 

 rather mineralogical character of the coast, and its popula- 

 tion, both as to kinds and numbers, varies correspondently. 

 Where it is rocky, it is inhabited by numerous gasteropo- 

 dous mollusks ; where muddy, or sandy, by burrowing 

 bivalves, or, in such localities, is not unfrequently devoid 

 of testacea. The common limpet, Patella vulgata, the 

 various species of periwinkle (Littorina), the dog-whelk, 

 Purpura lapillus^ certain forms of Trochus and Rissoa, 

 the little Skenea planorhis, the common mussel and the 

 minute Kellia rubra inhabit this zone on hard rocky ground 

 universally in the British seas. Local forms and occasional 

 stragglers from lower regions are here and there mingled 

 with them. On sandy and muddy shores numerous bivalves 

 are often thrown up by the waves, not a few of which are 

 to be found alive in the lower division of this zone. In 

 places where the water is brackish, it swarms with Rissoa 

 ulvte. It is capable of being divided into several sub-regions, 

 each marked by prevailing forms of animals and plants. 

 The uppermost is distinguished by the presence of the 

 smaller varieties of Littorina rudis and L . Neritoides ; a 

 second belt by the abundance of Mytilus edulis^ and the 

 larger forms oi Littorina rudis ; a third by the prevalence of 

 Littorina littorea and Purpura lapillus ,- a fourth, and 



