VI.] INTRODUCTION, ciii 



his valuable researches on the Invertebrata of the ^Egean 

 and our own Seas enabled him to define these zones 

 with considerable precision. Professors Loven and 

 Sars, as well as Oersted, have made us acquainted with 

 the range and limits of marine life in the Scandinavian 

 seas. 



In framing any scheme for dividing the sea-bed into 

 separate areas of molluscan habitability, according to 

 their depth_, it must be borne in mind that the extent 

 and produce of these areas vary greatly, and depend upon 

 the inclination and mineral nature of the coast. That 

 part of our sea-bed which is circumscribed by the line 

 of soundings may be divided into four distinct areas or 

 zones, of different width and depth ; and I will endeavour 

 to define briefly their limits, nature, and contents. 



The first is the Littoral zone, or the shore, which 

 fringes every part of our coast and lies between tide- 

 marks, being laid bare when the tide retires. Wherever 

 the coast is steep and rocky, this zone is very narrow. 

 Where it shelves gradually and is sandy (each of these 

 conditions being probably consequent on the other), the 

 strand frequently extends seaward for a mile or even 

 further. Where it is composed of cliffs, such as chalk or 

 boulder- clay, the beach is pebbly, and its width is 

 usually intermediate between that of the two other cases 

 I have mentioned. The pebbles are derived from the 

 wearing-away of the cliffs, either in the course of their 

 original elevation above the sea-level (which in many 

 cases appears to have been slow and gradual), or else by 

 the combined action of the atmosphere, rain and frosty 

 or of the tide and waves. This pebbly beach is sometimes 

 succeeded by a belt of larger stones or boulders, and 

 that again by a strip of sand, mud, or clay, as we advance 

 to meet the tide. In each of these cases the nature of 



