SCHARFF : ORIGIN OF LAND AND FRESHWATER MOLLUSCA. I43 



Roebuck, and Adams, and are being continued by others, enable us 

 to determine the exact range in the British Islands of every species 

 now living there. On the other hand, Messrs. Kennard and Wood- 

 ward have devoted a good deal of attention to the study of the past 

 range of these species in our islands. 



Now, among the three species I mentioned as having a very wide 

 range in our area, Hyalinia cellaria has occurred in various Pleisto- 

 cene deposits, but the other two — Helix nemoralis and Limncea 

 peregra — have even been discovered in beds of Pliocene age. As 

 regards their distribution beyond the British and Irish area, Helix 

 nefnoralis occurs from Southern Spain and Portugal to Scandinavia, 

 and from Central Italy to Northern Germany, but it is absent 

 from Russia, Eastern Hungary, and Turkey. Hyalinia cellaria has a 

 still greater distribution, being found all over Europe and Asia Minor 

 as well as on the Atlantic Islands and in Eastern North America ; 

 while Limncea peregra is likewise found in the whole of Europe and 

 besides in Western Asia and Northern Africa. The range of these 

 three species is continuous over the whole area which they inhabit. 



The continental range of the three local species, Geonialactis 

 fnaculosus, Helix obvoluta, and Dreissensia polymorpha, is very 

 different. The first only occurs in Northern Spain and Portugal. 

 Helix obvoluta is essentially a Central European species, ranging from 

 Hungary to Denmark, and in the west just crossing the Pyrenees. 

 Dreissensia polyniorpha is a typically Eastern species, but appears to 

 have spread from there through Russia and Germany as far as 

 England and France. None of these three have occurred in English 

 Pleistocene or Pliocene deposits. 



We are not justified in establishing generalisations from so few 

 examples as those quoted, but it seems as if species with a wide 

 range in the British Islands were more ancient and consequently had 

 also a wider continental distribution than those which have a local 

 range in our islands. This hypothesis is probably correct in most 

 cases, but it cannot be relied upon as being always applicable. 

 Among the ancient types of molluscs a certain number no doubt 

 persist, but others would gradually become extinct in certain areas, 

 leaving patches of small colonies in specially favoured localities. We 

 ought to find the remains of such species in Pleistocene and Pliocene 

 deposits, but as everyone knows, our geological strata do not give us 

 a complete picture of the fauna of past ages, and many recent species 

 are almost certainly ancient, of which we possess no fossil remains. 

 In such cases " discontinuous distribution " as shown by Geomalacus 

 maculosus undoubtedly indicates that we have to deal with ancient 

 vanishing types. Then, again, we have to take into consideration 

 that certain species among our land and freshwater mollusca are 



