ON BRITISH PULMOXI FERGUS MOLLUSC A. 131 



and fresh-water shells from the south of England may thus be 

 found on the ballast-hills on the banks of the Tyne. Gene- 

 rally the shells are found dead ; but we find Helix carthusiana 

 living in Northumberland, having been introduced among bal- 

 last. Our marine fauna has been sadly vitiated by the same 

 cause. Fallacies in judging of the distribution of shells may arise 

 in consequence of the mixing-up of fossil with recent species. 

 Where the fossils belong to the older strata, or even to the 

 crag, such a mistake is not likely to occur ; but where they be- 

 long to the Pleistocene period it is extremely difficult to distin- 

 guish. This more especially applies to marine shells : indeed, 

 several undoubted Pleistocene fossils have found their way 

 into the catalogues of living British mollusca. But it may also 

 happen in the case of land and fresh-water species. In a 

 Pleistocene bed at Portrush, in the north of Ireland, as has 

 been noted by Mr. Smith, there are many species of land shells 

 fossil, in such a state and in such a locality, that a person un- 

 aware of their history would, without hesitation, have enume- 

 rated them as natives of the place where they are found. It is 

 remarkable, that among them there is not a single specimen of 

 Bulimus acutus or Helix ericetorum, now abundant alive in 

 the immediate neighbourhood of the bed. 



All these modifying influences being taken into considera- 

 tion, it behoves us to be very cautious how we judge of the in- 

 fluence of geological and climatal causes on the distribution of 

 a species. The absence of a hill, a wood, a lake, or a ditch, 

 may cause the absence of many species, and lead us to attri- 

 bute their non-appearance in the district to a climatal cause, 

 when the presence of the necessary modifying influence might 

 have called them forth. It is only by a comparison of many 

 districts, and of the face of the country in each, that we can 

 hope to arrive at just conclusions ; and it is necessary in every 

 case to ascertain as far as we are able the circumstances under 

 which each species is found in other parts of the world, espe- 

 cially in Europe, ere we can argue fairly on its distribution ni 

 our own country. 



II. — On the Distribution of Pulmoniferous Mollusca in the 

 various provinces of the British Isles. 



Dividing the British Isles into ten zoological provinces, 

 we shall find that each presents certain features peculiar to 

 itself as regards the Pulmonifera inhabiting it. These peculi- 

 arities arise from the predominance of some one of the influ- 

 ences which I have enumerated. It would be very desirable 

 to consider the distribution in all the districts proposed by 



K 2 



