ScHARFF — On the Origin of the Eurojjean Fauna. 465 



in the Russian boulder-clay of organic remains, though the free- 

 swimming laxvEe of Arctic mollusca must have been present in this 

 sea which deposited the boulder-clay. 



Throughout the German boulder-clay we have evidence that small 

 colonies of molluscs were able, here and there, to find sufficiently 

 sheltered localities, where, perhaps, for a few generations they could 

 survive the discomforts arising from the turbidity of their new home. 



Prof. Jentzsch (43) discovered in Eastern Prussia no fewer than 

 ninety such localities of shells ; but in the majority of them he found 

 Arctic, North Sea, and freshwater mollusca, equally mixed. The 

 mean thickness of the boulder-clay is about 200 feet, and more than 

 half of this consists, according to this author, of stratified aqueous 

 strata. Prof. Jentzsch (p. 669) thinks that the occurrence of fresh- 

 water shells points to the existence of islands free from ice during the 

 diluvial period. Prom the point of view I have adopted, viz. that 

 the boulder-clay is a marine deposit, it seems to me that the occur- 

 rence of freshwater shells, along with marine forms, indicnles changes 

 in the salinity of the J^orth European Ocean. When the waters 

 became more brackish, many purely freshwater species would migrate 

 to the ocean from the coast ; and, as at the present day, Tellina halthica 

 and Cardium edule live side by side in the Baltic, with such fresh- 

 water forms as Limncea lagotis, L. ovata^ and Neritina fluviatilis, it is 

 not impossible to suppose that Vahata piscinalis, Paludina diluviana, 

 and Breyssensia polymorpha, which were the principal species found by 

 Prof. Jentzsch, actually inhabited the ocean for some time during its 

 existence. 



The occurrence of Dreyssemia polymorpha in the German boulder- 

 clay is particularly interesting. It suddenly makes its appearance in 

 the German lower boulder-clay, but then entirely disappears again, 

 and is not known from the subsequent more recent deposits.^ In the 



1 This species is unable to exist in water containing more than a certain 

 percentage of salt. It does not even occur in the southern part of the Black Sea, 

 and, of course, is quite absent from the Mediterranean, the Atlantic, and the 

 German Ocean. It is now found in canals and slowly-floAving rivers in Northern 

 Continental Europe and in England. It is supposed to have suddenly appeared in 

 England in 1824, and though the late Dr. Gwyn Jeffreys (42) denied that there 

 was any foundation for such a supposition, it is still quoted in text-books as an 

 artificially introduced species. So deeply rooted, indeed, is that belief, that even 

 the recent discovery by Mr. Woodward (94, p. 342) of some specimens in a post- 

 Pleistocene deposit in London, fifteen feet below the surface, has not effected a 

 change, and conchologists persistently cling to the favourite hypothesis which 

 offers such a vast field for pleasant speculations. It is almost certain, however 



