IxXXVi INTRODUCTION. [CH. 



channel. This last was called "Bissoa scabra" although 

 it was not the species so named and described by Philippi. 

 Bulla Oonc&wand other " northern" forms also occurred 

 among these Mediterranean shells, but under names 

 distinct from those which British conchologists have 

 given to them. If we can divest our minds of the popular 

 or received impression, that the diversity between species 

 which inhabit the extreme northern and southern por- 

 tions of the European seas is both general and well- 

 marked, we shall not be surprised at the discovery that 

 many species of Mollusca which at present bear different 

 names (such as Mitra Grcenlandica and M. ebenus) are 

 really the same or undistinguishable from each other, or 

 that even the Astarte incrassata of the Mediterranean is 

 only a variety of that polymorphous and northern species, 

 A. sulcata. 



The testaceous Mollusca of our own seas have been 

 separated by Forbes and Hanley into no less than nine 

 different types viz. Lusitanian, South British, Euro- 

 pean, Celtic, British, Atlantic, Oceanic, Boreal, and 

 Arctic. The limits of these so-called types have not 

 been defined with any degree of precision ; and, although 

 the proposed division is highly ingenious, it can scarcely 

 be considered as justified by the present state of our 

 information on the subject. It seems to me, after a 

 long and careful study of the question, that no more 

 than two groups (which are apparently distinct from 

 each other) can be recognized in a geographical point 

 of view ; and for these I would suggest the general, but 

 not inappropriate, names of "Northern" or North- 

 European, and " Southern " or South-European. It is 

 extremely difficult to fix the limits of even these com- 

 paratively wide areas of distribution ; but the " facies " 

 of each group is manifest to some extent in the lit- 



