106 DISTRIBUTION OF MOLLUSKS. 



most characteristic species were the Cerithium 

 vulgatum, Trochus crenulatus and spratti and 

 Cardium ewiguum. Between ten and twenty 

 fathoms numerous species of soft mollusks, allies 

 of Doris and Aplysia, abound. Some of these 

 nudibranchia have their soft slug-like bodies 

 painted with the most vivid colours, pure blues, 

 yellows, crimson, and green, excelling the most 

 gorgeous flowers, the form of whose blossoms 

 are in a manner represented by the delicate 

 petal-like plumes of their breathing organs. This 

 vivid colouring is peculiar to the mollusks that 

 live in the zones nearest the surface, for when 

 we go to great depths both shells and animals 

 are of pale or dusky hue. 



Below twenty fathoms we found a great part 

 of the species of the upper zones replaced by 

 others, often curiously representative in form. 

 In many cases the succession of representations 

 was kept up by three and even four species 

 of a genus replacing each other in regions of 

 depth. There was no transmutation of one into 

 the other ; each appeared with all its characters 

 precisely defined ; and usually before the cha- 

 racteristic species of a region had declined to 

 the minimum in the number of individuals, its 



