256 BRITISH MOLLUSKS. 



ferous raollusca have come from the same quarter." It does not 

 appear to me necessary to bring forward any geological phenomena 

 to acconnt for the universal distribution in the British Isles, of 

 plants and animals which were and are distributed universally on 

 the Continent in the same latitude. The Germanic portion of the 

 Coutiuent surrounds all the Continental side of Britain. 



§2. 



Land and freshivater species of opposite hemispheres are not always re- 

 presentatives, but are sometimes identical. 



Species of one province are not unfrequently simulated at dis- 

 tant points of another province by other species. These are re- 

 presentatives. Some of the British Lymncece, for example, are 

 represented with a remarkable degree of parallelism in North 

 America, in the midst of a fauna which in other respects is as re- 

 markably distinct. L. limosa has a very near representative in L. 

 catascopium, L. auricularia in L. macrostoma, L. stagnaUs in L. 

 jugularis, L. palustris in L. elodes, L. desidiosa in L. truncatula. 

 The evidence of the existence of indigenous land and freshwater 

 species in North America, which are identical with indigenous 

 British species, is very limited, but it is unimpeachable. Out of 

 our hundred and twenty-eight species, ten inhabit the United 

 States, but six of them, Limax agreslis, Zonites cellarius, nitidus, 

 and radiatulus, and Helix aspersa and nemoralis, are not indige- 

 nous. They have been conveyed accidentally in casks and other 

 packages, and have become acclimatized chiefly among the outly- 

 ing islands, and in the vicinity of the maritime cities, one species, 

 Z. cellarius, having spread to some distance inland. The remain- 

 ing four species, which I take to be indigenous in the United States 

 as well as in Britain, are Pisidium obtusale, Paludina vivipara, 

 Helix pulchella, and Ziia subeylindrica. The Pisidium is consi- 

 dered by Mr. Temple Prime, who describes it under the name P. 

 ventricosum, to be a distinct species, but Dr. Baudon, our latest 

 authority on that genus, asserts that it is identical with P. obtusale. 

 I will only speak then of the range of the remaining three. Palu- 

 dina vivipara lives abundantly along the south shore of Lake 

 Michigan, in Indiana, Illinois, Missouri, Arkansas, Alabama, 

 Georgia, South Carolina, and Florida. Helix pulchella, says Dr. 



