v.] INTRODUCTION. IxXXvii 



toral or shallow- water species, and especially in such 

 conspicuous and striking forms as those of Trichotro- 

 pis and Necera in our northern seas and Haliotis and 

 Galeomma on our southern coast. Taking the wider 

 basis of the European seas, I am not aware that any 

 species of Conus or Ringicula is found living in the 

 North, or that any species of Margarita or Lacuna in- 

 habits the South. It is, however, not unlikely that when 

 the sphere of our observation has been enlarged, and 

 a complete concordance obtained between the species of 

 Testacea from different parts of Europe, the exceptions 

 from a general distribution will become fewer and at 

 last disappear, and perhaps that only one common area 

 may be hereafter recognized. The distribution which at 

 present exists must be referred to a past state of things. 

 There can be no doubt that the area of diffusion was 

 formerly much more extensive than it is at present, and 

 that it has been restricted by subsequent causes. 



Reverting, however, to the proposed scheme of distri- 

 bution by Forbes and Hanley, as well as to the sugges- 

 tion now advanced by me, our marine Testacea may be 

 classed as follows : — 



1 . Northern ; 



2. Southern; 



3. Oceanic, or occasional visitants. 



The first of these divisions corresponds with the "Arctic" 

 and "Boreal" types of Forbes and Hanley, and the 

 second to their " Atlantic " and " Lusitanian " types. 

 Their "South British," "European," "Celtic," and 

 " British " types indicate mixed or neutral ground, and 

 partake both of northern and southern characters. The 

 third division answers to their " oceanic " type," but it 

 can hardly be regarded as indigenous to the British 

 seas. 



