34 IXTEODUCTION TO CONCHOLOGY. 



species prefer gravel ; otliei's^ a fine sand ; otliers, again, con- 

 ceal themselves in nmd. Limestone rocks are especially 

 resorted to by various species of the genera HelLv, Claii- 

 sUia, Bidimus, and others. 



In di-edging the ^gean Sea, Professor E. Forbes disco- 

 vered eight well-defined zones, each characterized by its pecu- 

 liar family of shells. The fii-st, termed by conchologists the 

 littoral zone, extended to a depth of two fathoms only, and 

 though in itself a naiTow belt, was tenanted by more than 

 one hundred species. The second region, embedded at the 

 depth of ten fathoms, was almost equally populous ; and a 

 copious Hst of species is given as characteristic of each 

 region down to the seventh, which lay between the depths 

 of eighty and one hundred and five fathoms. Deeper still, 

 appeared an inliabited space included in the eighth province, 

 where no less than sixty-five species of Mollusca were dis- 

 covered. Of these, the majority, beyond the influence of 

 light, were equally white and transparent. 



We might speak more at length concerning the marine 

 deposit of bright shells on the coasts of Cayenne, with those 

 of Madeira and the Straits of Magellan. We might mention 

 that cimous specimens of Tnchotro^pis have been found at 

 Icy Cape ; and that the 21urex trunculus, the Ti/rius Murex 



