404 THE AQUARIAN NATURALIST. 



just now is to trace from these majestic heights, a 

 kind of chart to guide us in our future wanderings. 



The marine Mollusca, which will next occupy our 

 attention, are generally mentioned by systematic 

 writers as inhabiting different zones or regions of 

 depth ; by which phrases are understood the several 

 belts or spaces margining the land or occupying the 

 floor of the sea, distinguished from each other by 

 peculiar features, dependent on the nature of their 

 animal or vegetable inhabitants. The working natu- 

 ralist knows well, that upon any given beach, the 

 objects he is in search of are by no means scattered 

 confusedly or at random, but are always to be found 

 in certain spots, the selection of their habitats agree- 

 ing with the nature of the ground, the depth of water, 

 and various other circumstances, which the practised 

 eye at once appreciates. So, on a larger scale, we 

 may divide the ocean into districts, all of which have 

 their appropriate or peculiar occupants. 



Now turn we to the sea, spread wide beneath us in 

 one broad expanse : 



" Its colours changing, as from clouds and sun 

 Shades after shades upon the surface run ; 

 Embrown'd and sombre now, and now serene, 

 In limpid blue and evanescent green j" 



and try to indicate and name the regions which it 

 will be found useful for us to distinguish in our 

 future excursions. 



The highest of these belts is the space between the 

 tide-marks indicating high and low water, an interval 

 of very great importance in the marine fauna of our 

 islands, and inhabited by numerous peculiar species 



