ON BRITISH PULMONIFEROUS MOLLUSCA. 133 



species which do not occur elsewhere in the British Islands, 

 and which may be considered indications of the positive in- 

 fluence of climate. They are Helix naticoides and Helix re- 

 velata. The total number of Pulmonifera, which I have ob- 

 served in Guernsey and Herm, is twenty-nine species. 1 doubt 

 not, on further investigation, half a dozen more may be added ; 

 but on the whole this district may be regarded as unfavourable 

 to the multiplication of species in consequence of structure, yet 

 favourable to the multiphcation of individuals in consequence 

 of climate. The fauna is nearly related to that of the opposite 

 coasts of France, if anything, a little more southern in cha- 

 racter. 



District II. — In the second district, the south-east of Eng- 

 land, the influences of climate and structure may be regarded 

 as equally balanced. Here we only find Helix obvohita and 

 Helix li?nbata, which, as well as Clausilia ventricosa, may be 

 regarded as climatal species. The only known localities for 

 Clausilia Rolphii are in this district. Such land-shells as 

 frequent chalky soils abound. In common with the south- 

 west division, it furnishes Helix pomatia, Testacellus halio- 

 toideus, and Bulimus montanus. The Helix pomatia is by 

 many accounted an introduced species ; but when we consider 

 the partiality shown by that shell for the newer calcareous 

 strata in all parts of Europe, and the geological correspondence 

 of its British and continental habitats, I think there can be but 

 little question of its indigenousness. The influence of a great 

 river, such as the Thames, is more evident in the presence of 

 the freshwater Pectinibranchia than of peculiar Pulmonifera. 

 Planorbis corneus is chiefly found in this province. Helix 

 carthusianella has not been found elsewhere in Britain. Some 

 of the more northern species, such as Helix scar bur geiisis and 

 Clausilia dubia, are absent. It is possible the absence of the 

 rocks of the coal formation may cause the absence of several 

 Helices and Pupag in this district. 



The Rev. Leonard Jenyns has favoured me with an excel- 

 lent manuscript list of the land and fresh-water mollusca indi- 

 genous to Cambridgeshire, one of the most northern portions 

 of this second province. The fauna of that county presents 

 some pecuharities, in consequence of the presence of large 

 tracts of fens, presenting features in common with the southern- 

 most part of the fourth district. The following analysis of the 

 localities of the Pulmonifera, enumerated in the catalogue of my 

 distinguished correspondent, will prove instructive: — 



The total number of undoubted species is sixty- two. Of 

 these five are Limaces, one Vitrina, one Succinea, twenty- 



