ON BRITISH PULMONIFEUOUS AJOLLUSCA. 135 



primitive structure of the rocks, doubtless, limits the number 

 of species, but is overcome by the climatal and marine influ- 

 ences as respects the number of individuals. Several of the 

 species too are decidedly southern and climatal in character, 

 such as Helix pisana and Testacellus haliotoideus ; others 

 are western, as Bulimiis acutus. The naturalization of some 

 of the exotic Bulimi, such as B. decollatus and B, clavulus 

 in this district, is a further proof of climatal influence. In the 

 more eastern portions of the district, such as Dorsetshire, we 

 find a general correspondence as regards species with those 

 of the second district, doubtless dependent on a similarity of 

 geological structure. Thus we find the western limits of 

 Helix lapicida, H. pomatia, Limneus auricularius , and Pla~ 

 7iorbis corneus, on the southern coast, on the confines of 

 Dorsetshire. The calcareous districts of Somersetshire and 

 Gloucestershire are more favourable for the production of 

 species, the catalogues of land and fresh-water mollusca in 

 those districts presenting a considerable increase over those of 

 Cornwall and Devon, dependent partly on geological causes, 

 and partly on the greater frequency of localities for aquatic 

 species. Throughout these counties the climatal influence is 

 equally evident. The lists of the th|ird province attain their 

 maximum in South Wales, evidently dependent on the pre- 

 sence of carboniferous strata in that locality. There, however, 

 climatal influence diminishes, and the Testacellus haliotoideus ^ 

 so characteristic of that influence, disappears. In that portion 

 of the district we find one of the two localities for the rare 

 Succinea oblonga, which has been observed elsewhere in 

 Britain only in the south of Scotland, in a district of similar 

 geological structure. In the south-western province we find 

 also one of the few British localities for another rare shell, 

 the Pupa cylindrica, which was found by Mr. Jeffi-eys, who 

 has done much towards the investigation of the mollusca of 

 this part of Britain, in the neighbourhood of Bristol. The 

 scarcity of aquatic species in the primitive counties of the 

 third province is not attributed solely to the scarcity of water, 

 but also to the nature of the sediment in pools and streams 

 of primitive countries being evidently unfavourable to the mul- 

 tiplication of Mollusca. 



District IV. — No part of Britain is richer in Pulmoniferous 

 Mollusca than the north-eastern division of England. The New- 

 castle list alone, thanks to the researches of Mr. Alder, enu- 

 merates sixty-seven species. That of Scarborough, a district 

 thoroughly investigated by Mr. Bean, exhibits no less than 

 seventy-four. The former is a carboniferous neighbourhood, 



