466 



Proceedings of the Royal Trkh Academy. 



loT^er boulder-elay, moreoTer, it becomes rarer as Tve go west, being 

 most abundant in Eastern Germany. Vhen "R-e take into consideration 

 that the origiQal home of the genus Dreyssensia, and of its ancestor 

 Congeria, is the Caspian (65), the natural inference from the fact of its 

 sudden appearance in Northern Germany alone seems to me sufficient 



6. — Map showing the land-connexion hetween Europe and Greenland during 

 Interglacial times, and the manner in which a way Mas opened to 

 Europe from Asia by the recession of the sea. The light parts 

 rej^resent the land at that time ; the shaded ones the sea. 



to conclude that there must have existed some communication between 

 the IN'orth European and the Caspian Seas at the time when the boulder, 

 clay was deposited. As we have seen in the preceding pages, many 

 other facts point to the same conclusion. Again, its non-appearance in 

 the upper boulder-clay and the more recent deposits of yorthem 



that Lreyssensia polymorph a suiTived in Xortheni Europe in some isolated lakes ever 

 since the deposition of the lower houlder-elay, and only spread with such rapidity 

 in recent years owing to the introduction of canals. The larva being a free-s^-im- 

 ming one, the species cannot exist in rapidly -flowing riveis. 



