INTRODUCTION. XXXI 



out an area on the continental coasts of the Celtic 

 province. 



The distribution of the four Pterojpoda that find a place 

 in this work is oceanic. Two of them are, however, as 

 yet unrecorded as extra-British, though it is probable that 

 they will be found widely diffused in the Atlantic, if indeed 

 they be not identical with the oceanic forms referred to in 

 our descriptions. 



The number of our species of testaceous marine Gastero- 

 'poda may be stated at two hundred and thirty-two. Out 

 of these a considerable proportion, not fewer than fifty-five, 

 has no place in any account of foreign Mollusca. It is 

 exceedingly unlikely that all of this number can be pecu- 

 liarly British, though in all probability many of them are 

 peculiarly Celtic. The members of some of our genera of 

 minute univalves, especially Odoslomia^ have not yet re- 

 ceived sufficient attention from foreign naturalists to enable 

 us to say, with any approach to certainty, whether they 

 are found on continental coasts or not ; and out of the 

 fifty-five species above mentioned no fewer than nineteen 

 are Odostomia. There are fifty-four species common to 

 the seas both north and south of Britain, mostly ranging 

 southwards to the Mediterranean, the few exceptions 

 ceasing on the coast of Spain, certain kinds of Littorina 

 and Patella especially. No fewer than fifty-two species 

 range to the southwards of the British Islands, but do not 

 occur northwards of them ; whilst thirty-four range north- 

 wards into the Boreal and European province, but do not 

 extend south of our area. Some twenty-eight or thirty 

 are inhabitants of arctic and boreal seas, and are common 

 to both sides of the Atlantic, within those provinces. The 

 few remaining are species observed in extra-British portions 

 of the Celtic province, but not elsewhere. 



